Without going in to it too much...technically you are correct because gel batteries are supposedly more robust.
However the problem comes from discharging one battery more than the other, therefore, either the least discharged battery gets fully charged and the most discharged doesn't see a full charge, or vice versa, and the most discharged battery gets a full charge and the least discharged gets cooked.
Gel batteries are much more sensitive to charge voltage than conventional wet cell batteries though.