I forget what the A is for but the GM is for glass mat. I was under the impression they do not off gas. At least I do not see any way mine can let any gas escape. I could be wrong. Check with Randy at BestConverter.com he is a forums member. I think Lewster also sells batteries and converters. The standard for AGM batteries is Lifeline. I bought a Universal which I found out after the fact is Chinese. There are also Optima batteries you can find at autoparts store but I think you may be able to do better.
I have an Intellipower 9245 and have been told it would be OK for AGM batteries. The thing is AGM batteries have a preference for particular charging voltages.
Sail on silver girl. Sail on by. Your time has come to shine.