Poor Akamaru
Hope he starts to feel better soon
Gastroenteritis is a bit of a generic term with many different causes so impossible to say how long it might take for him to fully recover. Hope for his sake it's a simple bug and the antibiotics do the trick.
I would defo claim on the insurance, it'll be classed as a pre-existing condition from now on in whether you claim or not so you may as well get what you're paying for!
I did a scientific test last year - I claimed for Riley (similar thing to Akamaru but it went on for ages and he got seen lots of times including being hospitalaised and an out of hours appt so cost loads) but not Lu as I think Lu only got seen twice so it wasn't much over the excess. Both boys are obviously same age, same sex, same postcode, been with them the same length of time. When the renewals came through they had both been raised by the same amount which was literally pennies.