Creating a Ruby gem that talks to another gem
I suggest you take a close look at how the gem you are using is tested. Those tests include mocks for all responses of the API (I assume) and is a good example for testing gems in general.

Notice:


helper methods in the support folder including shared examples
mock data to simulate responses from the source API
custom matchers to make code better readable and reduce repetition of code (DRY-principle)


Those to me are the three most important you should embrace for now.