Ocean Swim

I did this race in 2009. I freaked about the ocean swim for months. It turned out to be the easiest leg of the tri. The heat and humidity of the bike and run should be more fearsome.
Simply put in the proper swim training, do a few OWS where you can, then hop into the ocean when you arrive in PCB a few days before the race and you'll be fine.

I have not raced in the ocean, but I have OWS in the gulf.  The water is typically very clear and the current mild.  I would recommend just getting some OWS in at a nearby lake.  Then maybe get to the race a day or two early to get in a short one in the gulf prior to the race.  Good luck.

I have done GCT a few times in the past.  the conditions can range to smooth as glass to 4 ft swells (don't mean to scare you but I have seen people run into the water to start the race to just run back out, my hb got seasick during his swim at IM FL one year, although a different time of year).  More often it is very calm. 
The water is very clear so that is a big plus.  The bottom is sandy, another plus.  For training, I'd take one or two long weekend trips in March/April and drive down from N AL and swim once or twice before the race.  It'll be a nice training weekend and you can check out the bike and run courses while you are there.
How to get over the fear...the practice weekends would help...just do it.  You say you are a decent swimmer, there are boats out there, the water is clear (unlike 99% of lake swims I have seen) so you can easily be seen if you have a problem.  With clear water you will probably not get hit, scratched, kicked and runover as much as a swim in a murkey lake.  You will be fine.  The gulf is just like a big lake anyway .

If you can't get to an ocean before the race, I would get to the race a few days early (if you can) and practice in the Ocean 5 or 6 times (like get in, get out, leave the beach area, and then come back for the second time). That way you can better mentally prepare yourself rather than having one long open water session.

Also, this might sound dumb, but it really works: PRACTICE VISUALIZING THE SWIM! Watch some races with big swells, just to prepare yourself for the unlikely case that you have big waves, and visualize yourself being nervous and taking some deep breaths (or however you calm yourself down when you are nervous) and then visualize HOW you can get over it.

Visualizing things, even if you haven't experienced them, can really do wonders for mental and physical preparation!!

Good luck and have fun!

Thank you all so much for your words of wisdom.  I really appreciate it!