Gog.com are selling DRM-free games, so there's no copy protection, Internet activation, mandatory launcher, etc. It used to stand for "good old games", but they also have new titles these days. Same parent company as The Witcher developers. There is a launcher, but it's entirely optional - you can just pay prices that are generally comparable to Steam and download the installation files for a game, which require no Internet connection at all (apart from some edge cases, e.g. a very small number of multiplayer games).
Gog-games meanwhile is a piracy site that redistributes these DRM-free installers to people who are not inclined to pay for the privilege. What makes them preferable to other sites is that you get the trustworthy installers from gog and do not have to fiddle with potentially malicious cracks yourself. They are also uploading to fast file hosts. One thing they are particularly useful for is preservation, games that are now delisted on gog.com and elsewhere, only available there if you have purchased them in the past. The rather decent licensed Back to the Future game from Telltale for example can't be bought anywhere anymore (since the license for the movie franchise was only granted for a few years), but it's still available in its most convenient shape on gog-games.
I've found this one:
gemini://geminispace.info/
Needs a client to access, of course. Basic, but functional. I found a general-purpose forum not too different from reddit or lemmy through it (and they decided to call it a BBS, because the Eternal September hasn't happened to Gemini yet):
gemini://bbs.geminispace.org/
To the best of my understanding (and it's highly limited, since I only just learned about this, so take everything with a grain of salt), what Gemini does is primarily limit what the client can do. No local scripts, highly limited markdown. The server side is not limited. You can write any complex code you want that works behind the scenes - but it still has to deliver static pages (called "capsules") to the end user. This series of articles explains the basic underlying tech and uses the example of a simple server to illustrate how Gemini works:
https://medium.com/erus-encodia/creating-your-own-gemini-server-part-1-what-is-the-gemini-protocol-cf497477c4d
And yes, forms are possible, even though there appears to be a somewhat widespread misconception that they are impossible. Please excuse the sketchy-looking IP address instead of a URL, this was the best resource I was able to find on this (and yes, I checked if this page is on Gemini - this appears to be not the case):
http://216.218.220.144/tutorials/sig-tutorials/misc/gemini-forms.gmi
Screenshot if you don't want to click on the above link: https://i.imgur.com/s2mL3bM.png
Disclaimer: This is two years old and I have not tried to implement it myself. Looks entirely plausible though.
According to the search engine linked above, there are 2420 domains and 1,854,666 individual pages as of yesterday. This is about comparable to the World Wide Web at the same time 1994, a number that grew to 10,000 by the end of that year; I wouldn't expect the same explosive growth from Gemini - the field has already been plowed, after all. Gemini Space is small, but not a ghost town.