there isn't a lot to go by for garbage man, but from the 3 issues, it seems his durability/regen/immunity to physical damage is on par with MT, or at least, there seem to be no meaningful difference. Its not clear if garbage man needs any external source to regenerate, and if we go with the pre-thunderbolts MT, he is a self-contained ecosystem and doesn't need external plant/moisture. In a purely physical confrontation, it seems like a stalemate, though some of MT's more esoteric ooze/plant control stuff might give him some options.
The big issue will be the fact that Garbage man has a human mind and emotions, and he has spent 3 issues being confused and nervous, if not outright scared of things like hunters or a wolf monster. While it is doubtful that even full incineration from MT would kill Garbage Man, I think it probably puts him down long enough to give MT the win, as we have seen it take Garbage Man a significant period of time to regen after being exploded.
While it is arguable even just seeing MT would be terrifying for GM (he was frightened by another monster he encountered), being human, he will be susceptable to MT's ability to enduce fear in people, especially given the "doubious lawyer" backstory he is being given, lots of guilt for MT's empathic nature to exploit:
It could be a long fight, but given GM's lack of any real means to hurt MT in any way (this might change in forthcoming issues though) and liklihood of GM experiencing fear during the fight, gotta give it to manny, I'll say 7/10, but I really don't know where those 3 wins come from...