Garry's Mod, also known as GMod, is a...well, mod of
Half-Life 2. You get to mess around with an almost infinite amount of objects in an almost infinite amount of maps. The
possibilities are
literally endless.
There's also a couple game modes, including but not limited to:
-Hide and Seek; exactly what it sounds like.
-Prop Hunt; A couple players try to blend in with the map by becoming various objects while the others try to find and kill them.
-Guess Who: Prop Hunt, but with player models and special abilities.
-Trouble in Terrorist Town (TTT): The OG Among Us with guns.
Deathrun: One player is "Death." The other players have to dodge various traps triggered by "Death," and try to make it to the end of the map.
Garry's Mod was popularized by
VanossGaming and his friends after they migrated from GTA V. It is still popular seventeen years after its release, and
probably will remain
that way until whenever Garry's Mod 2 comes out.