It's not because they're "immature" or anything like that, and it's not a diss on anyone who still likes them, it's just that I tried to watch a new video of theirs and it just didn't hit me like it used to.
It's similar to what happened with me and the Nostalgia Critic (Tho admittedly with significant differences): I was a pretty big fan early on and for several years, but over time just fell off, started watching them more sporadically, before kinda just not keeping up with the new stuff anymore.
The only time I have ever seen game grumps was when them playing danganronpa popped up on my feed. I've known they existed for like a long time but never bothered with their content.