What can you tell me about their refund policy?

You can refund any purchase within 30 days. There are no restrictions on playtime, since GOG can’t feasibly control that.

You can get your refund either as store credit or to the payment method you used, however store credit is usually processed much faster.

Are the results on ProtonDB just as reliable for GOG versions as they are for Steam versions of games?

I can only vaguely answer this by saying the games that I tried (mainly indie games like Dead Cells and One Step from Eden) worked flawlessly.

Heroic Games Launcher has a Mastodon presence, so you can ask them there! https://mastodon.social/@heroiclauncher