Brawling is a skill, described in Basic Set, p. 182, and referenced in GURPS Martial Arts, p. 55, comparing it to Boxing and Karate, and GURPS Mysteries, p. 110, on its usefulness to investigators.
Sean Punch has commented on Brawling in the following posts to the SJG GURPS Forums:
- On its lack of a default 
- Compared to Boxing and Karate 
- On its advantages over untrained DX rolls , 
- On the justification for damage bonuses 
- On the lack of stacking damage bonuses with Boxing and Karate 
- On its not including any grappling 
- On its use in pistol-whipping 
- On what counts as a "fist load"