The mutation of the GECK would have been done by whoever the main badguy is.

They would have found the Vault of the Idiots, became its leader, took the GECK to one of Houston's MANY medical research facilities, broken open part of the case, and then blasted it with radiation mutating the seeds and crap inside of it mutated.

or something similar to that

How about they just tried to use the normal GECK but the radiation in the city mutated it to that mutated forest.

The vault people didn't know how to use it or even what it was, they are idiots.

Also that would be disappointing because there really wouldn't be a final boss or anything, it would be like Fo3 where you just there, flip a switch and its over.

My way provides us a final boss that would be REALLY op.

OK, I was thinking the eco-freak could have made it but he didn't intentionally mutate it.

But he was right in front of the GECK when the forest sprung up so he was the most mutated and came to love how the mutated forest destryoed or mutated everything it came across.

hmm I guess

that would make it less blatantly evil

Yeah.

It should be an awesome boss fight.

Lots of everything. Lots of options. Lots of customizability (character, armor, weapons). Setting up camp, property, buisness, big area, big cities. Fallout 4.