Yes, you start shooting at each other, if no hp is taken, there should be a console command for this.

Afaik, the custom collision system in Garry's Mod will cause bullet tracers to go through entities that are set to not collide with each other. While I can make the gamemode listen to mp_friendlyfire, it'll have to also lose the custom collisions at the same time which will cause various issues with players potentially getting stuck in each other at certain segments.

See the following issue here about the collision problem: https://github.com/Facepunch/garrysmod-issues/issues/3324

This is why I have lost all faith in humanity.