Valve recommends these configs:
Cable: cl_cmdrate 30 cl_updaterate 20 rate 9999
DSL: cl_cmdrate 30 cl_updaterate 20 rate 7500
Put those in console in depending on weather u have dsl or cable.
Hope it works
And ps. If it dose work either put those back in every time u start gmod or put it inside your autoexec.cfg
Deadsilent