Just upgrading to Windows 8 wont get ride of all the old drivers youll need a freash install so here are the steps that should help you with that
Step 1:ok if you still have the case that the Gateway came in or maybe even the documentation you should be able to find the CD-key for your windows 7 copy it's usually a sticker of some type (if you cant find your case or documentation you can download a program that will find your CD-key in your current installation of windows 7 once you write that down you can do the fresh install of windows with out having to buy Windows 8)
Step 2: find a copy of windows 7 online and burn that to a CD (if you don't have a CD drive or CDs you can do the fresh install using a USB Im not 100% on how to do that)
Step 3: once you have that you can install your new Motherboard and do a fresh install of Windows 7 (make sure you back up all your files)
hope this helps if you have any questions I might be able to help