drmike: An example of this I guess is Fallout New Vegas:
https://www.gog.com/game/fallout_new_vegas_ultimate_edition I see the GoG provided tag saying it has controller support but there's nothing under the Minimum system requirements section.
I don;t have an answer unless, since this example doesn;t have a recommended system requirements section but support is still provided.
Maybe someone else can offer a suggestion.
If not, you may want to contract staff directly:
https://support.gog.com/hc Sorry couldn't help more.
I figure it simply means that the game supports gamepads, but they're not
required. Does GOG ever list gamepads among the system requirements? I think the most I've seen is "a gamepad is strongly recommended". As far as I know, there are very few games on GOG which strictly require some kind of gamepad.
To OP: If a game is a multi-platform release ( also available on consoles ) and no more than say 10 years old, chances are that it will have gamepad support. For older games it can be hit or miss.
By the way, if you're considering to buy specific games, maybe someone here will know for sure whether they have decent gamepad support.