The Powerwise 28115 charger is a ferroresonant transformer type charger and its charge profile is based on the attached recommendations from Trojan Battery for their Lead-Acid Deep-cycle Wet-Cell batteries.
Basically, it starts out in a constant current mode (18A - 20A) until the on-charge voltage climbs to about 42V and then the amps start decreasing as the on-charge voltage increases. As the amps decrease to about 5A as the on-charge voltage approaches about 45V and the charger shuts off.
The max recommended charging amps for AGM is roughly 1/2 of what the max recommended charging amps for a wet-cell is and the 120AH batteries are roughly 1/2 the capacity of the 225AH wet-cell batteries the PW 28115 is designed to charge, so the initial amps (constant current mode) will be about 4X what your 120AH batteries can tolerate.
Also, the max recommend finish voltage for a 36V AGM battery pack is 44.1V and the PW 28115 shuts off at 45V +/-1V. However, you might be able to lower this to 44V or less by adjusting the pot on the control board. Of course, the AGM batteries will probable be cooked before the on-charge voltage gets that high due to the 4X higher than recommend amps.
Pages 19 and 20 of attached Trojan Battery user's guide have the charging profiles for Wet and AGM batteries.
Here is a website with some chargers that will work with AGM batteries, but not sure if they'll charge a 120AH 36V pack.
http://www.cartsunlimited.net/chargers-36-48v.html