There is very little research on this, and the gel product is designed for vaginal use. Some side effects, like those flu-like symptoms, are likely to be the same regardless of where the drug gets absorbed from. Upper GI side effects might be reduced with the gel. Lacking better data you should assume similar absorption, and thus go for a similar dosage (in mg) to an oral dose.
There are other pouchitis meds, though, that are much better studied than all this guesswork. Xifaxan (rifaximin), Tindamax, and augmentin come to mind. I’d try those before inventing new approaches. Adding a strong bacterial probiotic (e.g. VSL #3 DS) might also help when the “go-to” drugs aren’t an option. Some folks are convinced that probiotics are useless while taking antibiotics, but my personal experience suggests otherwise.