Taken from the Readme included:
yo kids, so i herd you wanna have fun.
well, extract the .bsp to your maps folder in garrys mod. and the BOXIN folder to your gamemodes folder.
the map is special, fall out of the ring, you die. each end of the ring has locker rooms, with health, switches, etc.
this is v0.4 of BOXIN!!
This also comes with a custom HUD. its intended for a server with multiple players. the Q menu is disabled (for no griefing)
NEW VERSION .5 with a new map and TWEAKS!! wohoo