An update for Garry's Mod has just been released. If your GMod installation hasn't updated itself, restart Steam.

Here's a list of the changes in update 69:

Added an initial 15 Steamworks Achievements
Fixed TF2 player and hat models being black
Fixed some server lagging exploits

Here's a list of achievements in this update:

• Play Singleplayer - Play singleplayer at least once
  
• Play Multiplayer - Play multiplayer at least once
  
• Startup Millennium - Start Garry's Mod 1000 times
  
• Secret Phrase - Say the secret phrase
  
• Addict - You have wasted a year of your life playing Garry's Mod!
  
• Map Loader - Play 20 different maps
  
• Play Around - Play a gamemode that isn't sandbox
  
• Friendly - Play around with 10 friends
  
• are you the real garry? - Play on the same server as garry
  
• Marathon - Play on the same server map for 8 hours
  
• One Day - 24 hours of your life wasted
  
• One Week - One whole week of your life wasted
  
• One Month - One whole month of your life wasted
  
• Half Marathon - Play on the same server map for 4 hours
  
• Killed 1000 Baddies - Kill 1000 Baddies

Try and get them all!