Here is the full list of all 29 Garry's Mod achievements. It takes around 200 hours to unlock all of the achievements on Windows.
Play singleplayer at least once
Experience 500 Lua programming errors
24 hours of your life wasted
Play a gamemode that isn't sandbox
Play multiplayer at least once
Play 20 different maps
Start Garry's Mod 1000 times
Say the secret phrase
You have wasted a year of your life playing GMod!
Killed 1000 Baddies
Play with 10 friends
Play on the same server as garry
Play on the same server & map for 8 hours
Play on the same server & map for 4 hours
One whole week of your life wasted
One whole month of your life wasted
Kill 1000 innocent animals
Eat 200 balls
Spawn 5000 props
Burst 1000 balloons
Remove 5000 things
Open the spawnmenu 100,000 times
Spawn 1000 NPCs
Spawn 2000 ragdolls
Kill 1000 friendly NPCs
Your Workshop uploads got 10 thumbs
Your workshop uploads got 100 thumbs
Your Workshop uploads got 1000 thumbs
Get 1000 thumbs on a single upload (then go to jail)