I didn't get any owners manual when I got my generic AGM battery from a co-worker that buys them by the case, so I don't know if this is applicable to the batteries from Gruber, but with the Odyssey battery, they recommend to charge it at 7-10 amps, I did this with my Odyssey, and it only took about 2 hours to charge it up.
Don't know if you want to try this method or not.