Arduino Leonardo/Micro As Game Controller/Joystick : 10 Steps - Instructables
Pro Micro Button Box with Matrix - Code Help - Programming Questions - Arduino Forum
Precision USB Joystick Controller BU0836
GitHub - norpchen/ArcadeStick: Arduino Leonardo based HID joystick / button panel with many features.
Turn your ProMicro into a USB Keyboard/Mouse - SparkFun Electronics
Create a Joystick Using the Arduino Joystick Library 2.0 : 12 Steps (with Pictures) - Instructables
Button Matrix Check and question about encoders - Programming Questions - Arduino Forum
I made a gaming control panel using an Arduino Leonardo to play simulators. Currently using it for DCS World, but it should work in most games just like any joystick / gamepad.