Hey Shendo,
I recently got myself a G751 recently and if the G75 is similar, one of the fans is to cool the CPU and the other is to cool the GPU, so if you're playing games that are graphically demanding, but not so demanding on the processor this may explain why one side is warm and the other is not.
In any case, I don't think it's anything to worry about as long as your temperatures aren't going sky-high.
Asus ROG G751JT-T7115H
17.3" IPS, Intel i7-4720HQ, 16Gb RAM, NVidia GeForce GTX970m, 256Gb M.2 SSD + 1Tb SSHD