I looked at Witcher 3 here and didn't see any mention. I've already got w3 on steam so I know it uses a controller but still, it is very confusing to do research on old games that use controllers.

I think they leave that up to the community with GOGMixes.

There is a Chrome add-on that shows this and some other things on the game page, but it's not consistent and not all games have the icons. Newer ones are more likely to have the info displayed.

Sadly I can't remember where it came from (just installed on my work computer and I'm on vacation), but hopefully someone will pop in here and mention it for you. Again, it's not great, but better than nothing.

More or less as usual, GOG is quite inconsistent and incomplete in its listing of controller support. Still, here is the available information.

Games that have the word "controller" under the Windows System Requirements:
http://www.an-ovel.com/cgi-bin/magog.cgi?ver=534&scp=gdspur&dsp=ipgfsorlcmbaxyzXh&ord=&flt=xcs~controller~&opt=&myf=ThuJul230335592015_gPWN6HyTvB7gq

Games that have the word "controller" under the Mac System Requirements:
http://www.an-ovel.com/cgi-bin/magog.cgi?ver=534&scp=gdspur&dsp=ipgfsorlcmbaxyzXh&ord=&flt=ycs~controller~&opt=&myf=ThuJul230337152015_xE7qMGVlLzCX1

Games that have the word "controller" under the System Requirements Compatibility Note:
http://www.an-ovel.com/cgi-bin/magog.cgi?ver=534&scp=gdspur&dsp=ipgfsorlcmbaxyzXh&ord=&flt=Xcs~controller~&opt=&myf=ThuJul230334252015_45909WY19Luba

Good luck.

What is your definition for a "controller"? Only XInput XBox360 gamepads, or also the older PS2-style DirectInput gamepads? How about joysticks, do you consider them as controllers? Like, say, playing Tie Fighter, Wing Commander or Descent Freespace with a joystick?

Is your request mainly for newer games (from the era where e.g. XBox360 gamepads already existed), or do you also want GOG to mark whether some old DOSBox game is playable with a "controller"? Do you expect them to be playable with a controller out of the box, or are you fine if you have to configure the controls yourself to a controller?

If a game was designed to be played with a keyboard + mouse, but can in theory be played (clumsily) with a gamepad too, should GOG mark it as having controller support? Say, most older PC FPS games. I guess you can configure them to be played with a gamepad, but it might be quite hard because they expect you to be able to react fast and aim accurately, something that is much harder with a gamepad. I guess that stresses what I was just saying. That list has also older games like Evil Genius (2004), various X-Wing and Tie Fighter games etc.

I presume at least the latter ones need some tinkering to be fully playable with a XBox360 gamepad (which is probably what most people are nowadays thinking with "controller support"), because such gamepads didn't exist back when those games were made. The games expect you to use an analog flightstick + keyboard. One might be able to configure the controls passably to a XBox360 gamepad, but I am pretty sure you'd still have to use the keyboard time to time (in order to change power distribution, sending commands to wingmen etc.).

Also, I think GOG has various games that sorta have gamepad support, but are expecting an older DirectInput gamepad. The default controls may work poorly on a XBox360 gamepad, but might be able to be reconfigured passably. Or not.

So... yeah they can be played with a gamepad at least in theory, but whether it really makes sense to do that is a different thing. So should GOG mark them as having "controller support", or not?

My example of Witcher 3 could work. It doesn't say anything about controller support. And yes, I was referring to an xbox 360 or xbox one controller. Gog obviously has many new games as well. It should be notated somewhere under the system requirements that a controller could also be used.

Lately I've just found that I enjoy using a gamepad more than being hunched over a KB for more than 20 minutes playing a game. Dont know why this has happened, maybe it's due to getting older and feeling nostalgic of my NES days.