Personally, I think Gel batteries are a poor choice for a conversion, for several reasons:
First of all, they require special care when charging. Charging a Gel battery on a "charge profile" meant for flooded batteries will destroy it. This also means that if you want to charge your house batteries from the main engine alternator while driving, you will need to set the charge voltage of the alternator down to a more appropriate setting. Some alternators with internal regulators are not user-settable, which might present a problem.
Secondly, they are about the same price, amp-hour for amp-hour, as AGM's, which can accept charge at a much higher rate, allowing a faster charger setting and minimizing generator run time. Lastly, AGM's can supply higher burst current.
The advantage of Gel batteries IF they are properly charged and cared for, is that they generally have higher cycle life than the other types.
Back when we were planning our conversion, I did an extensive analysis of life cycle cost for various types of batteries, and Gel came in dead last. (AGM won, hands-down.)
FWIW and YMMV.