Download Source SDK 2013 Singleplayer.
Then use this (this is VBCT preconfigured to mount garrysmod and compile off of the source sdk 2013 singleplayer bin files):
What it is: https://developer.valvesoftware.com/wiki/Batch_Compile_Tool
All mappers should be using this, but you have to manually configure your paths, as seen in the SDKPaths.pro. You could change it to HL2 paths instead of garrysmod and source sdk 2013, but I use Source SDK 2013 to map directly off garrysmod. There’s a tutorial right here: http://forum.facepunch.com/showthread.php?t=1374587
EDIT: For the record, VBCT is god, and once you use it, you never go back. Pro Tip: Advanced Options > Advanced Compile Options shows dozens of options and variables you can configure with VVIS, VRAD, etc, (it’s pretty much checkboxes) and it’ll automatically add the options onto the compile programs. It also doesn’t freeze up your computer to the point of super-lag. I can even play a game while compiling with VBCT, and still not have any stuttering.