How to use opatch to check if Oracle Gateway for DRDA is installed

Log on to your oracle server, and type

cd $ORACLE_HOME/OPatch
./opatch lsinventory -oh /full/path/to/oracle_home -details > /home/oracle/inventory_details.log

The top lines of the generated file will reveal what's installed in your ORACLE_HOME:

cat /home/oracle/inventory_Details.log | more

Result:

Installed Top-level Products (2):

Oracle Database 12c                                                  12.1.0.2.0
Oracle Database Gateways                                             12.1.0.2.0
There are 2 products installed in this Oracle Home.

After that, more details about your installation is listed:

Installed Products (137):
Oracle Database Gateway for DRDA                                     12.1.0.2.0
Oracle Database Gateway for ODBC                                     12.1.0.2.0
Oracle Database Gateways                                             12.1.0.2.0