Loving itch.io as a Linux gamer this is adding even more games to my growing library (and backlog). A lot of sites and services do no offer what I am going to suggest but I hope that itch.io would be the first.
I use STEAM, GOG, Humble Bundle, Bundle Stars, GameJolt, Yoyo Games and others.
All of these inform users that their games have gamepad support.
I've been looking for some more details - not in terms of which brand of gamepads are supported but what button layout does the game support.
2 button gamepads - directional + 2 action buttons
3 buttons gamepads - directional + 3 action buttons
6 button gamepads - directional + 6 buttons
twinstick gamepads - with x buttons needed