Gel Cells are rated in Ah or "amp hours". This is how many hours the battery will power 1 amp of constant drain. Or, conversely, how many amps you can run on the battery for 1 hour.
15 Ah = 15 amps for 1 hour or 3 amps for 5 hours etc.
I've found Ah ratings to be somewhat overconfident. Voltage drops as power is depleated and towards the last few amp hours of life lights will dim or devices may give "low battery warnings".
It all depends on how much $$$ you are willing to spend and how much weight you want to carry. Some gel cell 12v batteries run 50 Ah. (but they weigh about 40 lbs.)