Taken from:
http://www.snarkpit.com/editing.php?page=tutorials&game=HL2&id=119
(By Leperous)

First, copy and paste the gameinfo.txt file (there should be one in your sourcesdkcstrike_sample_content directory) to your half-life 2/hl2 directory. Open it up; set SteamAppID to 220. I also recommend you remove all the references to CS.

Load Hammer, and create a new game configuration (I replaced CS:Source otherwise it'll annoy me asking which config to use each time I load a map), call it HL2 or something.

Use the base.fgd as the only FGD file (found in sourcesdk/bin directory); there is currently no "proper" Half-Life 2 FGD.

Set the game executable directory and game directory to be your half-life 2hl2 directory, and the hammer map directory to wherever you keep your maps (e.g. half-life 2hl2maps). Now close and re-open Hammer so it can load up the relevant textures.

Note that this will also allow you to place HL2 models, sound effects etc. However, I don't think that the current base.fgd has all the entities you can use in it, so you still can't do some things. It is also more memory intensive than CS:Source mapping seems to be!

Use this gameinfo.txt if your having problems:
http://rabidmonkey.bluewolf72.com/files/GameInfo.txt