What is a gel battery rated to temperature wise?
The drawback with most batteries is heat and gel cell batteries are no exception. Heat is one of the fastest ways to cut the lifespan of your battery short.  Although they perform well at cold temps.  The lead gets oxidized with lead sulphate insulation at higher temperatures. Specs are not widely published, yet drivers in hot climates expect battery life of 1~2yrs.  Although your question is more about the leakage rate and this depends on contamination due to the ageing of the electrolyte.