Thank you for the quick reply. I would mark this question solved, however, dimensionally I believe the card will fit.
I opened up the case, measured and it seems to have enough room LxWxD wise.
My concern is if it will clear the motherboard completely. Because of the length of the card past the motherboards end it may hit where, I think what's, a 4-pin wire is connected. I may be able to push the wire out of the way but the card closest to the motherboards end and top of the 4 pin may touch.
Has anyone used the Radeon R9 390 Nitro in the Gateway FX6860-UR20P. Or with the stock motherboard for the Gateway FX6860-UR20P?
Thank you again for your help!