Just so we don't confuse anyone, the Bosch and Optima are both AGM batteries (not gel cells). I don't think there are any major manufacturers selling gel batteries for automotive use. Some motorcycle manufacturers supplied true gel cell batteries in new bikes 10-15 years ago, but they were not reliable. The gel battery would be unable to start the engine if it was not well charged because the gel electrolyte becomes stratified (after cold start and short trip, for example), but would be fine after sustained charging.
Gel cells are better suited to low current standby use like small security systems where they are normally kept on float charge for extended time and rarely need to supply current. Gel cells typically need higher float voltages than conventional lead acid flooded or AGM batteries. The relatively low float voltage of the DC-DC converter in the LEAF is ideal for AGM and OK for flooded batteries, but would likely not keep a gel battery adequately charged.
I am sure either the Bosch or Optima AGM battery will work out well for you.
Thanks for the explanation. I've used gel cell batteries for years, mostly in security and fire alarm applications where they are constantly charged just waiting for a power outage, they can also be used in all positions(don't have a liquid acid like the AGMs) so I just figured the AGM batteries where larger versions of the little 6ah 12v gel cells I've used, sounds like not.
I went looking for a replacement 12v battery today at a local "farm" type store. They had a couple different brands of AGM batteries but not in the 51R size, the only battery in the 51R size was a flooded lead acid for $115. I passed and will check at a couple other local stores for an AGM and if they are too expensive locally I'll probably just go the Optima route from Amazon. Several years back I replaced the 12v battery in our Prius with a yellow top Optima from Amazon and it's worked flawlessly ever since. The Prius originally used a flooded lead acid but as it was mounted in the passenger compartment was a special battery with a vent tube that ran underneath the car to vent any acid fumes. The replacement AGM didn't have to be vented which made it an ideal replacement for the Prius. Of course, our Leafs wouldn't have to be remotely vented but I'd still prefer a battery that didn't produce acid fumes when charged.