There is no official ESRB rating for GMod, but it is built using T for Teen games. Single player is probably ok, but you never know what kind of wackos you get online. I would be worried more about the complication level more than the violence. GMod doesn't have a campaign or story mode. It requires you to build your own fun. If the child is under 10, they might not have a very good time playing without help.
actually, the games that this runs on are rated m.(intense violence, blood and gore, and language on at least one of those)
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No Ofcoures not!
no. gmod is only for the pc.
No. Gmod is only for PC
steam/steamapps/username/gmod/gmod/data/adv_dup(or something like that)/* *place dupe .txt file or file folder with dups in it, here)
if your talking about the gmod that you buy off steam, nothing game wise oh, and $10