In practice I never went away from GIMP for most Photoshop-like tasks. Which isn't a signal that GIMP is great, but that I grew used to its deficiencies many years ago. And it has gradually improved: It has single-window UI now, after all.

For the two very most common bitmap editing tasks(color remapping and selection-based edits) it's OK and it even boasts some fancy features these days. For drawing, it has a workable set of brushes and fills, though I think Krita is the better app here now. For graphic design things(typography, layout, layers of procedural effects) too many things still involve a slow, indirect workflow, but there are signs of this problem being chipped away at.

(The single best tip I have for GIMP is: get in the habit of using the "/" key quick search instead of using the dropdown menus)