Firstly, they are similar. For example, Orcish warriors are something in between a demon and a swordsman, albeit a bit larger than the former and much larger than the latter.
Secondly, the description of Gremlins in Heroes 5 clearly states that mages created them as an alternative to the goblins that had escaped. It is also added that they are a good alternative, as Gremlins are significantly smarter.
That's exactly why Gremlins did not originate from goblins, as I'm saying, but were merely created to replace them. However, we will no longer see Gremlins as units. Perhaps that's for the better.
Демократия есть одурачивание народа при помощи народа ради блага народа.(Уайльд Оскар)