Gells, not necessary!
Just be sure that all your batteries are of one technology, and prefereably the same size. I started with wet cells of various types, then moved to gells, and eventually came back to wet cell, 6 volt golf car batteries. They give by far the best bang for the buck, and you will get the best prices from a golf car dealer. Gell cells have some attractive features (won't short underwater!, don't sulphate as quickly when left discharged, have a lower self'discharge rate, and can be mounted in different positions) but have some disadvantages apart from much higher cost. They are much more heat and voltage sensitive and easier to ruin. They have a smaller capacity for a given size. Although they are always described as having a faster charge rate than flooded wet cells, none of the manufacturers' engineers would confirm that to me. Additionally, when questioned closely, all admitted that for tropical cruising, gell cells are too heat sensitive. My own experience confirms that.I also do not think they last for as many cyles. I would stick with the much less expensive wet cells. And if I was just itching to spend money on batteries, I would consider AGM technology, which seems pretty promising. Perhaps someone else has experience with that. There have been several discussions on this site that might be illuminating, regarding batteries.