What's GMOD? (horror game)

I’ve seen a couple of different examples on YouTube, but the theme is the same: a scary looking face (basically a large 2-D cutout) is chasing you and if it catches you you’re dead. A few questions:

Is this an application of the “Garry’s Mod” Half-Life 2 sandbox, or am I conflating two different things?

Does the face always know where you are, or can you lose it if you get out of sight and hide? Can it always get to you no matter where you are or do obstacles/ height differences thwart it? If there are weapons in the game can you fight it?

GMOD is the standard shorthand for Garry’s Mod. I assume you’ve been watching people play on one of these maps:

The ones that have been turning up the most on my YouTube feed feature the Obunga and Selene Delgado Lopez faces.