Games like XNemesis
Games like XNemesis include Garry's Mod, Ravenfield, Ancient Warfare 3, Just Cause 2, and People Playground. We matched them by the tags players give XNemesis on Steam (Action, Indie, Shoot 'Em Up, Sandbox, and Moddable): closest matches first, and among equally close ones the better-known games.

Garry's Mod
Shared: Sandbox · Moddable · Shooter

Ravenfield
Shared: Action · Sandbox · Shooter

Ancient Warfare 3
Shared: Action · Sandbox · Indie

Just Cause 2
Shared: Action · Sandbox · Shooter

People Playground
Shared: Sandbox · Moddable · Indie

Hard Bullet
Shared: Action · Sandbox · Indie

Arma 3
Shared: Action · Shooter · Sandbox

Grand Theft Auto V Legacy
Shared: Action · Shooter · Sandbox

Blockstorm
Shared: Action · Indie · Shooter

BONEWORKS
Shared: Action · Sandbox · Shooter

Grand Theft Auto IV: The Complete Edition
Shared: Action · Moddable · Sandbox

KovaaK's
Shared: Action · Indie · Shooter

My Friend Pedro
Shared: Action · Indie · Shooter

Crysis 2 Remastered
Shared: Action · Sandbox · Shooter

Freeman: Guerrilla Warfare
Shared: Action · Shooter · Indie

Grand Theft Auto V Enhanced
Shared: Action · Shooter · Sandbox

Overgrowth
Shared: Action · Indie · Sandbox

Blockland
Shared: Sandbox · Action · Indie

Squirrel with a Gun
Shared: Action · Sandbox · Shooter

Hot Dogs, Horseshoes & Hand Grenades
Shared: Sandbox · Shooter · Action

RUNNING WITH RIFLES
Shared: Action · Sandbox · Moddable

Blade and Sorcery
Shared: Action · Sandbox · Moddable

Revolution Ace
Shared: Action · Shoot 'Em Up · Indie

BONELAB
Shared: Action · Sandbox · Shooter