have you tried reinstalling garry's mod and unsubscribing from all garry's mod addons? you can try disabling all except the perpheads content first before you uninstall everything.
This happened to me ages ago, once connected I'd be able to walk for a second and then it would crash my whole game. I've upgraded pc since then but I reinstalled GMod and it had fixed it. Don't know if it will be the same for you.