I don't have a Gulfstar... BUT... You are going to have to do your own math based on your tank size and shape. Mine is 7.06 gallons per inch per tank. But if you need any ideas dipsticks, I did a thread on them a while back:
Good Dipstck Material
As for cleaning, look up a professional tank cleaner (NOT A FUEL SCRUBBER) in your area. he might be able to do it without an access panel. I had access panels on this thread, but the same guy did a great job on our new boat just thru the filler cap. However, we were on the hard and he was able to use the travel lift to tip the boat around to get our new tanks equally as clean.
Skinny Dippin's Tank Cleaning: A Trip Report