So if you use G8 to temporarily reduce a ships speed will it negate jerjerrods ability which affects a ships current speed ?

G8 says "before" determine course set. Jerry's description says "during" the determine course set. Therefor the G8 reduces the speed temporarily and that becomes its "current" speed for the determine course action. Therefor Jerry can use its ability even if G8 was used against the ship.

Keep in mind that when the g8 lowers speed it may make Jerjerod not needed, which is why the timing matters (and is good for jerry)