The best replacement for gmod is jbmod.

The best alternatives to gmod are rated as following:
★★★★★  jbmod
★★★★  sfm
★★★★  minecraft
★★  tf2
★★  smod
★★  roblox

Your alternative for gmod is not listed? Suggest!
Speaking about gmod, one probably shouldn't forget about hamachi.

Other search terms related to gmod and their ratings are:
★★★★★ hamachi
★★★★ hl2
★★★ half life 2
★★ counter strike source
★★ portal
★ tf2
★ halflife2
homelessgoomba
kinect
You have in mind a companion for gmod and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of gmod in all years. Click on graph for more.
gmod
jbmod
sfm