My dad just tried to fit an optima battery in his GNX-----long story short---he could not find one to fit and has had to go another route.He actually had a couple and physically tried---to no avail.
I had a Red Top and now a Yellow with no mods what-so-ever. It is a TIGHT fit but it sits in there fine and doesn't move around at all. Not sure what "size" the battery is but so far I am 2 for 2 with no problems at all.