GSM requires knowing the details of the Request Type and Data (JSON) to be sent with each request and also parsing the response / messages sent back. It only has a few actions, but they can be used to make a variety of the available GS requests with properly user formatted request type and JSON data sent.
You must have a GameSparks account and project to test. Set apiKey and apiSecret to GS object properties in the example before testing.
For debug, make extensive use of browser console to review messages and responses between client and server.
If you are starting fresh w/ cloud game services and C3 runtime, I recommend Playfab and the Master Collection instead of using this plugin.