No, they are not gel cells. They are AGM type batteries. AGM is "Absorbed Glass Mat", the battery is constructed with a spiral wound fiberglass mat as a separator between the positie and negative plates. The acid electrolyte is absorbed into the mat.
Advantages: The battery is sealed and will work in any position, so the danger of acid spillage is eliminated, unless you puncture the case. Also, Optima claims that the spiral wound cell construction makes the battery more resistant to vibration. As you may know, bits of lead can flake off the plates of conventional batteries, short cuicuiting the cell.
I specified Optima batteries for an application at work. We needed batteries in the turret of an M-109 self-propelled howitzer (big 155 mm cannon on a tracked chassis). Lots of shock and vibration and close to the crew, so we wanted a very reliable battery that would not leak or outgas.
Now unless you are setting up a trail rig that will see lots of heavy duty use, a good conventional battery will serve you well. My truck sees mostly highways with Jersey-sized potholes. I use an Interstate Megatron Plus battery and it has been excellent. Interstate, Optima, and Walmart MAXX are all made by Johnson Controls.
You can find an excellent tutorial and brand name to manufacturer cross reference at:
http://batteryfaq.org
Bob