I'm from Massachusetts and grew up listening to the same stuff that a lot of the "Mass Metalcore" bands listened to, and God Forbid always seemed to be the New Jersey version of that vibe.
I first saw them at the 2002 NEMHF (and chatted with Byron after their set) and I think they were higher up the bill than Lamb of God at that time (LoG was actually pretty terrible back then). I think In Flames took them out once or twice, Anders was a very big fan and used to wear God Forbid shirts onstage back when he did that sort of thing.
However, I didn't know anyone who called themselves a big GF fan. A lot of people knew them and liked them, and they got on enough tours for a while to sustain a scene presence, but yeah when the scene thinned out it just wasn't there anymore for them.
Remember that back in the metalcore heyday, MTV2 had the remade headbangers ball and they played a TON of these bands in that 2002-2005 period - anyone with a video on there was making it. When that ended, it slowed down the cash flow for a lot of these bands..... and the "trendy" kids went in different directions (either deathcore or the emocore stuff).