Florescent shop light kicks off GFC wall outlet
I would swap the GFCI with another one, and see if the problem repeats. Then replace either the light or GFCI, accordingly. 

No fluorescent fixture should ever trip a GFCI, if it does, it's the ballast, either due to being really cheap, or having a fault.  Given that ALL garage outlets must now be GFCI, I can't see how anyone could get away with selling a fluorescent fixture that trips GFCIs. 

Like Machavity says, replacing an old one isn't a bad idea.  And the best reason is the march of technology: New ones are better.  Just don't lose that advantage by getting a bottom-shelf junk unit.