The gel cells are basically lead acid car batteries (in this case with 2 cells instead of the usual 6). In this application you can use pretty much anything that can supply lots of current for a long time.

The tricky bit will be the charger. If the flashlights are the ones with a built in charger, and you want to use that charger, then you should stick to gel cells. If you are willing to charge them externally, you can use pretty much anything.

Attempting to charge LiPo or even Nimh with a lead acid charger is likely to require a fire extinguisher, or even an ambulance.