1) Start Garry's Mod and set it to the lowest settings so your computer can still run smoothly.
2) C: Drive --> Program Files --> Steam --> SteamApps --> Common --> Garry's Mod
3) Create batch file called Start1.bat
set /a var=1
echo Started %var% times
set /a var+=1
Copy paste this into a notepad file, and save it as a .bat file
4) Create the second batch file called Start2.bat
timeout /t 7
taskkill /f /im "hl2.exe"
Copy paste this into a notepad file, and save it as a .bat file. NOTE: The timeout/t 7 can be changed to a lower number if Garry's mod can start and get to the menu quickly enough.
5) Save both in the Garry's Mod folder.
6) Run both of them
Sidenote from SgtFopper (April, 10th 2013):
This method is not working atm. A new method is in the works)
1) Create a shortcut file (GMOD.lnk) to Garry's Mod in your Steam folder along the lines of...:
E:\Steam\Steam.exe -applaunch 4000
2) Now open up Notepad (or equivalent) and create a batch file like this and save it into your Steam directory as gmod.bat:
ECHO "Starting GMod Millenium Trap"
ping -n 75 127.0.0.1 > NUL 2>&1
TASKKILL /f /im "hl2.exe"
3) Double click on the batch file and go afk for a LONG time
NOTE: Edit the ping time (the number 75 above, in seconds) to suit your PC spec - as long as you reach the main menu the counter will have gone up!