To sum it up as quickly as possible, at last check (as in, I haven’t touched my GM since the new hotfix, and anything is always subject to change) Green Monster does two things when it’s stacking. It builds Corrosive damage up to a hundred percent (it’ll look like 75 percent on dummy because flesh) in a short time, and it builds splash damage by a hundred percent in…like a minute.
If that second part sounds familiar, it’s because that’s Blast Master. So, GM runs a Blast Master at the same time, as long as you meet stacking conditions. Here are the stacking conditions for Most Guns (I’ve been told some guns are weird and I can’t test them all).
Green Monster starts stacking once you tap the trigger. On a Jakobs, this will fire one bullet. On a Maliwan, it’ll probably just try to charge. To verify this works on your gun, go melee the dummy. You’ll see Corrosive damage building up.
A reload of your weapon will set you back to zero and will not restart stacking.
Your next trigger pull will restart the stacks. This means: most Jakobs guns can not fire a rouns affected by GM, because you need a new trigger pull each time.
An impractical use of this would be to fire one shot. Let your bonuses stack, then only use grenades and bear. (Bear will not get the Corrosive damage. But he will get the splash).
A practical use would be to use a fully automatic gun, and make sure you don’t reload right before getting in iron bear.
I’m very tired, so, if you have more questions. I gotchu. But also, my Bust-A-Bear Workshop thread has a demonstration of this…before I knew it was blast master.