I don't have any experience with Gatekeeper, but it sounds like you've already done some of your homework. Although it's been too long to remember, I played with Laravel 5.2 (a year ago?) and liked their approach to authentication. It might be worth checking into, since you're looking at auth libraries.
The difficult thing to do is make an authentication library truly framework agnostic, because it means:
1) The library doesn't use the framework's sessions, database connection, etc.
2) You've got foreign code to look through to figure out how everything works, because if you need to make customizations (and you will) then you need to know how it works.
3. You've got to spend time figuring out how to have that library use the framework's sessions, database connection, etc.
Honestly, I didn't make my auth for anyone else, even though I'm sharing it. Now in my 12th year of playing with auth, I know it well enough to make special modifications anytime I need. That's the best part of making your own.
Community Auth places most of the code in a library and a model. Because the library is autoloaded, it runs the constructor well before anything happens in the controllers. Controller routes (controller + public methods) call on methods that are placed in Auth_Controller (a base controller). That's just my way of doing it. In essence, the foundation of Community Auth is three files. Four files if you count the Tokens library.
