I had my fiberglass tub/shower unit refinished and there is a gap between the tub/shower unit and the drywall. I've read some info that you can use silicone caulk for gaps that are about 1/8". But some areas around the unit, the gaps are larger (between 1/4" and 1/2"). Can caulk still be used or is there a better solution?

You can caulk it with this
http://www.dap.com/newsroom/press_releases/KwikSeal3.aspx
And even if it doesn't crack or shrink it will yellow over time. So does the " Tub caulk" that you buy over time.

To do a perfect job, I would get a small tub of drywall compound and fill the gap with it first.

Then a primer sealer Kills or Binzs over that.
A touch up paint would be nice but its now sealed and the caulk you apply there will not shrink.

You can caulk the 1/2 first and do a partial there and re caulk over that and the 1/4" gap on the second run.

The compound/primer and touch paint/caulk is the best way to now you can run an 1/8 bead to seal those areas