"Sadly you will need someone with more sleep than me to fix this as I'm too tired to think of much"
If that's what your going to post, then why post at all.
This is a prebuilt desktop, correct? Upgraded parts are not guaranteed to work since manufacturers tend to use modified parts which prevent any further upgrades. It may be compatible according to their specs, but considering the fact it is a prebuilt desktop with modified hardware ect. It is not compatible. If using your old card now works and everything else is left the same and it works, then it is a problem with Gateways modified hardware. I have attended many of these issues when users recommend cards for prebuilt desktops and they give them the ok that it will work, but when it comes to the time it doesn't. This is not the case for all pre-built desktops, it is only a matter of the card and manufacturer of the desktop itself.
All the best, Unknownofprob.