You will most likely need to use the "Source Engine 2009" version of Source SDK and choose "Garry's Mod".
Before running "Source SDK", navigate to your:
"C:\Program Files\Steam\steamapps\youraccountname\sourcesdk\bin\orangebox" folder.
(I am assuming you installed Steam using default installation settings which tends to be C:/. If Not, go to your other installation but follow the same folder path ways.)
After navigating to the orangebox folder, locate a file called "GameConfig.txt".
Save this file in an alternate location you are able to locate in the future.
Download this new "GameConfig.txt" from here -
http://jump.fm/GPQLQPaste this new config into the orangebox directory and overwrite your original GameConfig.txt
Open up Source SDK, select engine version "Source Engine 2009", and then select "Garry's Mod" under the game selection.
Open up Valve Hammer, and go into the options menu. Simply replace any .fgd directory locations and also the compiling tools (ex. VBSP, VVIS, VRAD) as well if you need to.
Use the following entities for player placement.
- info_player_start.
For Props, Make sure the models you are using are in fact compatible with the entity you choose, ex: prop_static, prop_physics, prop_dynamic.
You can easily check if the model you select in the model viewer is compatible by opening up the "Info" Tab on the model.
In this case, the chair model can be used as a prop_static or prop_physics. Generally, you may still encounter errors if you choose to make it into a prop_static.
Go with prop_physics and have the model "sleep" and "frozen" which you can alter in the parameters of the entity.
Finally, if you want to have CS:S models, make sure in the options menu of Valve Hammer, you include the counter-strike source .fgd file in addition to ttt.fgd, base.fgd, etc.
I hope this helps! :D
If you encounter further problems along the way, post here again and not only I but the rest of the community will be happy to assist you.