True. It's sad.

There are multiple factors involved:

1) In general, communities have drifted away from traditional forums like this. Discord is the most active community for GC gaming I am aware of.

2) Often Steam Discussions Pages are more popular, not necessarily true for GC3.

3) GC3 Modding was kept off of steam workshop, which, in this dark and modern era, led to GC3 Modding essentially being dead on arrival (It's a niche product in a niche genre, and the presence of ANY steam workshop content, without mods, drives traffic away from the only rational alternative (Nexus Mods) thus subdividing a very small audience pool. That being said, at it's height there were maybe 5 modders of "note" at a given time. I'm not trying to dis anyone, I'm just saying the mod-author pool is extremely small, and I believe virtually all of them have moved on from GC3 (myself included).

4) The game is rather stale and old at this point, especially since the sequel is well underway. This understandably draws attention away from the old game in most cases.

These are the major factors involved, and I hope I have answered your question as to "where is the community"

TLDR: Try Discord. That's where the hip kids live nowadays.