Many more tutorials exist for Photoshop, but I would imagine you may be able to translate them to a Gimp workflow without much effort. Since you're experienced with the Gimp and design work, this might be a wash. There are 4 main things that Photoshop offers: Much like switching from Windows to Mac or vice-versa, your mind has been trained a certain way and there is a cost to re-training. Don't forget that most of the people who say Photoshop is 'easier' or 'more intuitive' likely started with Photoshop and are conditioned to work that way.

This is typically the largest barrier to using it - users who have been conditioned to the Photoshop-style interface - and you are already over the hump. As a many-year use of The Gimp, no doubt you are familiar with its peculiarities and interface. It all comes down to your needs and your particular skillset. Don't ever give more prominence to the tools used than to your own abilities.

If your Gimp workflow is efficient and produces good art, what does it matter if there are a bunch of Photoshop tutorials or downloadable brushes available?.

In the absence of an immediate need that Gimp isn't meeting, there is no inherent 'learn Photoshop' rule that you must adhere to.

If you have extra room in your budget, it never hurts to learn another tool.

Discounted photoshop