I was a Photoshop user from the late 90's until about 3-4 years ago. It was a slow transition to FOSS tools, because there was never a concerted effort to switch to FOSS, it just happened because I like the tools better, and it's easier to not have to deal with licensing.

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.