Is it easy to transition from GIMP to Photoshop?

If I become fluent with GIMP, will transitioning to Photoshop be difficult/confusing? Or are they pretty much nearly-identical?

I know that they aren’t exactly the same, but I guess I just want to know if GIMP is a good program for learning the fundamentals of photo-editing before switching to Photoshop. I don’t want to become a GIMP expert, switch to photoshop and have to relearn everything…in that case I’ll just buy Photoshop.