What is garrys mod rated?

It depends on what the player does i guess

But its mostly what Half Life 2 is rated

If the player has counterstrike source also installed

the content (Characters and guns) gets transferred to garrys mod

so they can play around with it in the game, it depends on a few factors

(1) what will they do?

(2) what other games that rely on the source engine do they have installed?

(3) what do you think is suitable for them

in garrys mod, they are only limited by their imagination, in Garrys Mod you can build pretty much anything, and i mean anything