Right, this is a long answer but it should fix you're problem.
I'm guessing your front audio is AC97 so use you're mobo instruction manual to figure out how to plug in the front audio jack (use the AC97 diagram, not the HD audio diagram), if you don't have the manual DL from the gigabyte page. Your case should have a manual telling you how the pins in your front audio jack are arranged, make sure that both manuals agree on which pins go where, or you have a serous problem.
http://www.gigabyte.com.tw/Support/Motherboard/Manual_Model.aspx?Cla ssValue=Motherboard&ProductID=2551&ProductName=GA-P35C-DS3R
I think you also need to set your front sound to AC97 In your bios, again read the manual on bios settings for that.
Right, that was the easy stuff, now for drivers, YOU MUST INSTALL THE Microsoft UAA Driver first, then the Realtech driver! If you're just installing the sound drivers with the driver CD that came with the mobo this shouldn't be a problem, if you're using the drivers from the website there's a folder with the Microsoft UAA Driver in it, gigabyte don't tell you where it is, but i'm sure you can find it if u look hard enough. Once you've got that done install the Realtech sound drivers and you're set.
Now the bit that had me stumped for a couple hours when i installed the sound on my gigabyte mobo. Theres a setting in the Realtech sound program that says "disable front audio detection" you must disable front audio detection if you have speakers or anything plugged into the back of you're case or no matter what you do no sound will come out of the front audio. There is also the option some where to have both front and back sound working, but clicking it never made a difference to me, i assume it's a error in the program. once you've done this you should be able to set the front jack to speakers or headphones or whatever you have plugged in on you're Realtech control panel, and the rear jack should auto detect properly, otherwise just set it manually.
Ok, this isn't the nicest explanation I've ever given, but i can probably do better once i get home from work where i can give you the folder that the Microsoft UAA Driver is in and exactly what menu to turn front jack detection off on. Plus i have all the manuals there (;
Hope this helps