To answer the question flat-out, I'd say "easygainers" are generally mesomorphs. They adapt quickly and efficiently to most forms of exercise. If you're talking bodyfat, though, then I'd say yes. Most "easyfatgainers" would be true endomorphs.
That being said, I've never been a big fan of the endomorph/mesomorph/ectomorph classification. I think you'll find that the overwhelming majority are some kind of mix, making the class systems mostly useless.
I think you'd be better of (and have less headaches) if you pretend you've never heard of those categories and keep doing what you've been doing; that is, learning what works and doesn't work for your own body, irrespective of what may or may not work for others.