I occasionally have GI issues too but am no expert, but unwittingly I followed both Carolyn and Nancy's advice for my first HM last week. I ate my prerace dinner early at 6pm (it was a light meal and not heavy on fiber), then got up early to--ahem--shall we say "lighten the load"
my breakfast was 1.5 hours before race and very light too, toast & coffee (must have coffee!), then a bite of stinger-waffle just before my warm-up. I was a bit earlier awakening on race day but everything else was very similar to my long-run training ritual.
One thing I would add is that I was careful to continuously hydrate in the 3-4 days before my race. I drank lots of water and herb tea. I think this helped keep things "running smoothly" and helped me feel comfortable while running, maybe that will help you, too?
Anyway, I hope you find something that works for you and that you enjoy your first half-marathon! Mine was one of the best experiences of my life and I hope yours is that for you!
A ship in the harbor is safe. But that's not what ships are built for.