You need to make below changes in bash_profile or you need to set these env variables manually. I took a backup of bash_profile and make these changes. Once work is done I will restore the bash_profile from backup
export ORACLE_HOME=/home/oracle/goldengate_12.3
export PATH=$ORACLE_HOME/bin:$ORACLE_HOME/OPatch:$PATH
export PATH=/home/oracle/goldengate_12.3:$PATH
export LD_LIBRARY_PATH = /home/oracle/goldengate_12.3:$ORACLE_HOME/lib:/lib:/usr/lib
[oracle@oel6-node03 ~]$ cat .bash_profile
# .bash_profile
# Get the aliases and functions
if [ -f ~/.bashrc ]; then
. ~/.bashrc
fi
# User specific environment and startup programs
PATH=$PATH:$HOME/bin
export PATH
# Oracle Settings
export TMP=/tmp
export TMPDIR=$TMP
export ORACLE_HOSTNAME=oel6-node03.localdomain
export ORACLE_UNQNAME=srcdb
#export ORACLE_BASE=/u01/app/oracle
export ORACLE_HOME=/home/oracle/goldengate_12.3
export ORACLE_SID=srcdb
export PATH=/usr/sbin:$PATH
export PATH=$ORACLE_HOME/bin:$ORACLE_HOME/OPatch:$PATH
export PATH=/home/oracle/goldengate_12.3:$PATH
export LD_LIBRARY_PATH=/home/oracle/goldengate_12.3:$ORACLE_HOME/lib:/lib:/usr/lib
export CLASSPATH=$ORACLE_HOME/jlib:$ORACLE_HOME/rdbms/jlib
once changes are made source the bash_profile
[oracle@oel6-node03 ~]$ source .bash_profile
[oracle@oel6-node03 ~]$ env|grep ORA
ORACLE_UNQNAME=srcdb
ORACLE_SID=srcdb
ORACLE_HOSTNAME=oel6-node03.localdomain
ORACLE_HOME=/home/oracle/goldengate_12.3
[oracle@oel6-node03 ~]$ which opatch
~/goldengate_12.3/OPatch/opatch
[oracle@oel6-node03 ~]$ opatch version
OPatch Version: 12.2.0.1.6
OPatch succeeded.
you should be able to do lsinventory
[oracle@oel6-node03 goldengate_12.3]$ opatch lsinventory
Oracle Interim Patch Installer version 12.2.0.1.6
Copyright (c) 2020, Oracle Corporation. All rights reserved.
Oracle Home : /home/oracle/goldengate_12.3
Central Inventory : /u01/app/oraInventory
from : /home/oracle/goldengate_12.3/oraInst.loc
OPatch version : 12.2.0.1.6
OUI version : 12.2.0.1.4
Log file location : /home/oracle/goldengate_12.3/cfgtoollogs/opatch/opatch2020-06-16_13-18-06PM_1.log
Lsinventory Output file location : /home/oracle/goldengate_12.3/cfgtoollogs/opatch/lsinv/lsinventory2020-06-16_13-18-06PM.txt
--------------------------------------------------------------------------------
Local Machine Information::
Hostname: oel6-node03.localdomain
ARU platform id: 226
ARU platform description:: Linux x86-64
Installed Top-level Products (1):
Oracle GoldenGate Core 12.3.0.1.2
There are 1 products installed in this Oracle Home.
There are no Interim patches installed in this Oracle Home.
--------------------------------------------------------------------------------
OPatch succeeded.
As per readme the required opatch version is (12.2.0.1.13). otherwise you will receive this error while applying patch. See the known_error section at the end of the doc.
so download the required opatch go to below link and select the opatch version you need and download

Once patch is downloaded replace the old opatch version from goldengate home with this newer version
[oracle@oel6-node03 goldengate_opatch]$ cp p6880880_122010_Linux-x86-64.zip /home/oracle/goldengate_12.3/
[oracle@oel6-node03 goldengate_opatch]$ cd /home/oracle/goldengate_12.3/
[oracle@oel6-node03 goldengate_12.3]$ mv OPatch OPatch_orig
[oracle@oel6-node03 goldengate_12.3]$ ls *zip
p6880880_122010_Linux-x86-64.zip
[oracle@oel6-node03 goldengate_12.3]$ unzip p6880880_122010_Linux-x86-64.zip
check version again
[oracle@oel6-node03 goldengate_12.3]$ opatch version
OPatch Version: 12.2.0.1.21
OPatch succeeded.
In our case we want to apply patch 12.3.0.1.191015 (30341552) on GG home. Note that our OGG version is 12.3 but database is 11.2.0.3. So select the patch carefully as per your env
Oracle GoldenGate 12.3.0.1.191015 Patch Set Availability (Doc ID 2637699.1)

Download the patch

Once downloaded unzip it. Make sure your env variables are set properly and Ensure that you shut down all the services running from the Oracle GoldenGate home.
[oracle@oel6-node03 30341552]$ env|grep ORACLE_H
ORACLE_HOSTNAME=oel6-node03.localdomain
ORACLE_HOME=/home/oracle/goldengate_12.3
[oracle@oel6-node03 30341552]$ env|grep PATH
LD_LIBRARY_PATH=/home/oracle/goldengate_12.3:/home/oracle/goldengate_12.3/lib:/lib:/usr/lib
PATH=/home/oracle/goldengate_12.3:/home/oracle/goldengate_12.3/bin:/home/oracle/goldengate_12.3/OPatch:/usr/sbin:/usr/lib64/qt-3.3/bin:/usr/local/bin:/bin:/usr/bin:/usr/local/sbin:/usr/sbin:/sbin:/home/oracle/bin
CLASSPATH=/home/oracle/goldengate_12.3/jlib:/home/oracle/goldengate_12.3/rdbms/jlib
Go to patch directory and apply the patch
[oracle@oel6-node03 30341552]$ opatch apply
Oracle Interim Patch Installer version 12.2.0.1.21
Copyright (c) 2020, Oracle Corporation. All rights reserved.
Oracle Home : /home/oracle/goldengate_12.3
Central Inventory : /u01/app/oraInventory
from : /home/oracle/goldengate_12.3/oraInst.loc
OPatch version : 12.2.0.1.21
OUI version : 12.2.0.1.4
Log file location : /home/oracle/goldengate_12.3/cfgtoollogs/opatch/opatch2020-06-16_13-40-50PM_1.log
Verifying environment and performing prerequisite checks...
OPatch continues with these patches: 30341552
Do you want to proceed? [y|n]
y
User Responded with: Y
All checks passed.
Please shutdown Oracle instances running out of this ORACLE_HOME on the local system.
(Oracle Home = '/home/oracle/goldengate_12.3')
Is the local system ready for patching? [y|n]
y
User Responded with: Y
Backing up files...
Applying interim patch '30341552' to OH '/home/oracle/goldengate_12.3'
Patching component oracle.oggcore.ora11g, 12.3.0.1.2...
Patch 30341552 successfully applied.
Log file location: /home/oracle/goldengate_12.3/cfgtoollogs/opatch/opatch2020-06-16_13-40-50PM_1.log
OPatch succeeded.
Verify the patch details
[oracle@oel6-node03 30341552]$ opatch lsinventory
Oracle Interim Patch Installer version 12.2.0.1.21
Copyright (c) 2020, Oracle Corporation. All rights reserved.
Oracle Home : /home/oracle/goldengate_12.3
Central Inventory : /u01/app/oraInventory
from : /home/oracle/goldengate_12.3/oraInst.loc
OPatch version : 12.2.0.1.21
OUI version : 12.2.0.1.4
Log file location : /home/oracle/goldengate_12.3/cfgtoollogs/opatch/opatch2020-06-16_13-41-39PM_1.log
Lsinventory Output file location : /home/oracle/goldengate_12.3/cfgtoollogs/opatch/lsinv/lsinventory2020-06-16_13-41-39PM.txt
--------------------------------------------------------------------------------
Local Machine Information::
Hostname: oel6-node03.localdomain
ARU platform id: 226
ARU platform description:: Linux x86-64
Installed Top-level Products (1):
Oracle GoldenGate Core 12.3.0.1.2
There are 1 products installed in this Oracle Home.
Interim patches (1) :
Patch 30341552 : applied on Tue Jun 16 13:41:21 IST 2020
Unique Patch ID: 23165952
Created on 23 Sep 2019, 22:44:57 hrs PST8PDT
Bugs fixed:
29261736, 29747634, 29705645, 27136510, 29507343, 29548820, 29429716
29184736, 26669140, 29254077, 29228446, 29194609, 28978464, 28966487
28507572, 29244155, 28137760, 29624366, 29449724, 29329967, 28962077
28317143, 25814949, 25826633, 29158717, 28961772, 28931837, 28925874
29135238, 28985480, 28570846, 28168478, 27526864, 27249136, 28729717
28715607, 28584037, 27630100, 29710864, 28669347, 28363292, 27713308
27220677, 27027216, 28436394, 28224916, 27412413, 28088324, 28514112
28584325, 28152293, 28300591, 27033034, 28285034, 25369310, 28085066
27798548, 27768111, 28343346, 28573688, 28280704, 27647881, 27819094
27078084, 28043789, 26869744, 28023549, 24411701, 27052326, 28074033
28011195, 28085411, 27731916, 27278457, 25496669, 27672044
--------------------------------------------------------------------------------
OPatch succeeded.
Known error
Opatch apply failed when patch applied with old opatch version
[oracle@oel6-node03 30341552]$ opatch apply
Oracle Interim Patch Installer version 12.2.0.1.6
Copyright (c) 2020, Oracle Corporation. All rights reserved.
Oracle Home : /home/oracle/goldengate_12.3
Central Inventory : /u01/app/oraInventory
from : /home/oracle/goldengate_12.3/oraInst.loc
OPatch version : 12.2.0.1.6
OUI version : 12.2.0.1.4
Log file location : /home/oracle/goldengate_12.3/cfgtoollogs/opatch/opatch2020-06-16_13-22-25PM_1.log
Verifying environment and performing prerequisite checks...
NApply could not load patch from location '/home/oracle/GG-12.3.0.1.0/patch/30341552'
UtilSession failed: /home/oracle/GG-12.3.0.1.0/patch/30341552/etc/config/actions.xml with java.lang.Exception: Input string of filterString() is invalid
Log file location: /home/oracle/goldengate_12.3/cfgtoollogs/opatch/opatch2020-06-16_13-22-25PM_1.log
OPatch failed with error code 73