If you want to test your client library, you’ll have to have unit tests, integration tests, functional tests. Gatling is not aimed to provide those tests.
If you want to test the load your client can handle, that is another subject. In that case, Gatling may help but with:
a custom protocol (to handle the call to your client instead of HTTP for example)
a mock server (to answer to your client)
The path will be Gatling → client → server