Just for clarification, Aseprite is "source-available": you're free to browse [1], clone and build the source for your own usage as long as you don't redistribute your build. There's even a tutorial for it. Official builds, of course, are paid.
Being able to distribute your builds and improvements is one of the essential freedoms of Free Software.
Photoshop is a bit a of jack of all trades, and the FOSS tools are usually more targeted. So Gimp may have all that you need, but that depends on what you use Photoshop for. I would look at the capabilities of not only Gimp, but Krita (for digital painting), Darktable if you want a more specialized tool than Gimp for photo manipulation, Inkscape for vector editing, Libresprite for pixel art (which is a fork of Aseprite, which went from open to closed source).
As far as being easy to transition to, I would say that it's a bit easier now than it has been in the past, but they make no effort to be a Photoshop clone. The biggest hurdle is re-training your muscle memory. I would suggest to do what I did, and do a project here and there using Gimp, and eventually the muscle memory problem will not be such a hurdle.