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Assuming the Gateway FX6800 - http://support.gateway.com/s/PC/FX/FX6800/FX6800sp2.shtml
Up to 12 GB of DDR3 1066 MHz SDRAM (triple-channel support on six DIMMs)
It's best to get a Tri Channel Kit of 3x__GB. The i7 9XX default is DDR3-1066, and 'most' OEM MOBO's lack DRAM/Memory Frequency settings in the BIOS. I would certinly boot into your BIOS and LOOK for Memory settings. Also, I've never seen any i7 9XX not run 3x4GB instead of the listed 6x2GB limit posted on Gateway's site. In fact many have been proven to run 6x8GB (48GB).
So boot into your BIOS and see if there's a 'DRAM/Memory Frequency' setting.