It was probably the "gold standard" modern port of the OG DooM/DooM II/Hexen etc games. There are some other source ports made for multiplayer, some made for cranking the graphics up to 11, some made to ensure exact match to original engine behavior for speed running purposes, but GZDoom was kind of the standard the community had settled on that did enough of all of that to be good.

There are full "boomer shooter" style indie games built off of it and the expanded features it offers, like 3D model support and greatly improved lighting. Then the lead dev went a little bonkers (I don't know the specifics) and people forked it into this and have kept improving it.