Are you looking for instructions on how to pull the RDS data down to the local database or simply link the gateway to a local MySQL in AWS?
If it is to link to the local MySQL then you can log into the gateway with the console key and type the command su - ssgconfig -> Select option 2) Display CA API Gateway configuration menu -> 3) Configure the CA API Gateway -> 1) Database Connection to modify the DB connection information.
Sincerely,
Stephen Hughes
Director, CA Support