Can I use a smartphone as a gamepad for playing desktop games, android games in emulator, games in another android/iOS phone? Is there any app(s) supporting this feature?
*www.slashgear.com/mobile-gamepad-app-turns-android-phones-into-windows-game-controller-28326651/
*www.instructables.com/id/MAKE-YOUR-ANDROID-PHONE-INTO-GAMEPAD/
You can try above methods but I am not sure if they will work for you.
These guys make awesome use of phone as controller - airconsole.com . These are party games, everybody connects to the same wifi and use their smartphone as the controller. Unfortunately most games are paid. But you can try a few free games just to see how it works. I had a blast playing this with 3-4 people. The game we played was stupid simple, but so much fun with family. This is a complete and closed platform that won't work with normal games.
So, the games are proprietary games of Airconsole