How to connect AWS RDS by using AWS Gamelift?

To read data from RDS from Lambda, you could use AWS provided libraries and methods. For instance if you are using Python boto3 would be the library that you could use. Here is some reference instructions and code that would help you read from RDS using a Lambda - https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/UsingWithRDS.IAMDBAuth.Connecting.Python.html