Almost certainly a RAM problem.
Make sure the modules are properly seated.
If the problem persists, test each RAM module separately with Memtest86+
Download ISO and create a CD from it with IMGBurn
Boot your Gateway from that CD and the RAM testing will start automatically and continues to run until you stop it with the ESC key. Let it run for 4 passes on each module, although any errors will be found usually in the first three passes.
(you can only test each module separately by having only one module installed at a time).