I guess technically by “definition” one could consider GIMP a knock off but that’s like saying an my and all image processing programs are knock offs of one another.
That said, GIMP isn’t being developed to take whatever features are in Photoshop and remake them into a free program. GIMP is developed entirely independently of Photoshop with very little regard given to what Adobe is adding to its product.
There is one thing of which you may be ABSOLUTELY CERTAIN: Adobe is a very large corporation with a team of copyright lawyers on call, if not on staff. If GIMP was, in any way, a copy, those working on it would get a “cease and desist” order faster than you can type “copyright.” They have not.
And as a former Photoshop user who now uses GIMP exclusively, I can testify under oath that it is not a copy, by any so-called "definition.” Such definitions are entirely specious—or you can use the short form:
Sorry for the rant, but this sort of thing pisses me off.