Is it possible to use AWS GameKit with custom game engines?

Yes. All the features are available in the AWS GameKit C++ core SDK and game developers can call these functions directly to use the Cloud Formation templates, deploy Cloud Formation Stacks and interact with the deployed GameKit back end features. So if a game developer wants, they can use the Cloud Formation templates and back end features in their custom game engine. Refer to the C++/Blueprint examples in the plugin to get reference on how to call the AWS GameKit C++ SDK.