Some of the history is right on the Patreon site or the video.
As I remember it, back in mid to late 1990s, there were various newsgroups going strong on Usenet in Strategy, RPGs, Simulation games, space games, adventure games, etc. In those topics, you'd see some overlapping people posting. I was there. Tom and Mark Asher were both pretty prominent. There were a bunch of folks at Gone Gold (like its founder Rich LaPorte, but also a lot of the forum community there) who were also frequent posters in the newsgroups. And, as Tom notes at Patreon, it was this setting where he and Mark founded the site here in 1999 under the CNet sponsorship umbrella, which promptly when belly up when Qt3 was founded.
The site originally consisted of a front page with reviews and features and links to Tom and Mark's work elsewhere. As Tom notes in the text at Patreon, Mark left fairly early on, seeing the writing on the wall with that whole internet bubble burst.
The original forum software (the Blue Boards, as they were called) was pretty terrible. It was threaded without major category breaks, and new posts didn't bump active topics to page one. You had to go searching for them. I think it was at least a year or two before Qt3 went to forums that we'd all recognize. I think when you see really, really old posts here with "Anonymous User" tags and stuff, those are posts that were archived from the old boards. As I recall, initially Qt3 didn't even have a forum at launch. The original idea was to post and link new content here and then discuss on the CSIPGs groups.
When folks originally began posting here, there were a few people who posted at Gone Gold and Qt3 both, but it also seemed like it forked the old Usenet/newsgroup folks. I had a couple of posts on the Blue Boards, but waited a year or so before registering here--I think it was during one of the first extended GG outages, when it started to become clear that folks like Bill Harris and Andrew kind of didn't have a lot of incentive or support for staying at GG or keeping it up and running.
I think it was also the rise of MMOs that drew those two communities together as well. Both DAoC and WoW especially.