Games like Spaceguy
Games like Spaceguy include Counter-Strike 2, HROT, Selaco, Warhammer 40,000: Boltgun, and DUSK. We matched them by the tags players give Spaceguy on Steam (Action, Indie, Retro, 2.5D, and Shooter): closest matches first, and among equally close ones the better-known games.

Counter-Strike 2
Shared: Action · Shooter · FPS

HROT
Shared: Retro · Indie · Action

Selaco
Shared: Shooter · Action · FPS

Warhammer 40,000: Boltgun
Shared: Retro · Action · FPS

DUSK
Shared: Retro · Shooter · FPS

The Citadel
Shared: Retro · Indie · FPS

DOOM + DOOM II
Shared: Action · Retro · FPS

KovaaK's
Shared: Action · Indie · Shooter

CULTIC
Shared: Action · Shooter · FPS

Maximum Action
Shared: Action · Retro · Indie

Fallen Aces
Shared: Action · FPS · Retro

Counter-Strike
Shared: Action · Shooter · FPS

Turbo Overkill
Shared: Action · FPS · Shooter

Wolfenstein 3D
Shared: Retro · Action · FPS

Devil Daggers
Shared: Indie · Action · FPS

MOUSE: P.I. For Hire
Shared: Action · FPS · Indie

Aimlabs
Shared: Action · Shooter · FPS

Project Warlock II
Shared: Retro · Shooter · Action

Bright Memory: Infinite
Shared: Action · Indie · FPS

Prodeus
Shared: Retro · Action · Indie

Quake
Shared: Action · FPS · Shooter

SUPERHOT
Shared: Action · Indie · FPS

PUBG: BATTLEGROUNDS
Shared: Shooter · Action · FPS

Serious Sam 3: BFE
Shared: Action · FPS · Shooter