Source SDK Windows Gadget supports now Source SDK Base 2013 MP and SP.
I made the update myself, and added the last source sdk base 2013 for Singleplayer and multiplayer.
Just download the file and double click on it.
Accept the installation the gadget. if the older version is running, close it before to install the new one.
When the installation is done,the gadget appears and you have to set your game configuration name and path.
That’s all !
I hope this will make the modding easier.