it has always been this way with GMOD, i remember when i first bought it YEARS ago on his website before steam. it has its ups and downs. somehow everyone always comes back. as soon as some YouTuber or someone starts making content they will come back. i think a lot of it was Fortnite and other games took away a lot of attention from it. if you dont want it to die then play, get friends to play. be vocal in the steam community about an update.