I’d recommend in no particular order:
Eat the Dungeon
Currently Unnamed Stuffing RPG
Some Bullshit (less weight gain, but the quality is exceptional)
Apostles (darker story, but one of the better narrative based games)
Vale City (a bit grindy)
a machine translated japanese game (think it tranlates to “Maid’s long vacation”: A new japanese weight gain RPG from @3GPG93kDyAAWbKS on twitter
Olive and The Ruby Bra
Seeds of Destiny (Pregnancy is the main fetish)
the Starbound mod “Big Fatties”
Above Gods
It has already been mentioned, but “Forks” is looking pretty good too
Keep in mind that all of these are female oriented (with the exception of the starbound mod, that is 50/50, but more slanted towards whatever sex your character is)