It used to lack non-destructive editing ("adjustment layers" in Photoshop parlance) until recently, it's a core foundation of editing workflows for designers and photographers, it lets you layer transformations of over immutable rasters. This was in Photoshop since 2005.

I've used GIMP and Photoshop for a very long time (close to 30 years).

In ~1998, GIMP was not quite as good as Photoshop 5 and was more awkward to use, but you could see how it could close the gap. It had impressive underlying tech that could handle large images on computers at the time. There was an expansive library of weird and neat plug-ins and scripts. It felt like we were at the start of a great shift in which OSS software would "catch up" and eventually replace desktop power tools, just as Linux had done with web servers. It was... the year of Linux on the Desktop!

By ~2005, GIMP was starting to really catch up to where Photoshop was in 1998, but Photoshop had added lots of quality of line features like adjustment layers and layer effects, way better text rendering, and amazing new features like spot healing brushes, vanishing point warping, etc. The gap was widening. But GIMP still did all the core stuff, and Photoshop was annoying users by shoving Adobe Bridge down their throat, etc. So people were still hopeful for a replacement.

By ~2012, GIMP was adding.. an awkward single window mode? It lacked tons of by-now-basic features that made it totally impractical for professional use. Photoshop, meanwhile, was adding amazing time-saving features like Content-Aware Patch and Move that seemed "magic" at the time. The tech gap was widening, but Adobe was also pushing subscriptions down users' throats, which was very unpopular, so GIMP still had a chance to make a come back.

By ~2018, GIMP was finally adding.. basic CMYK support for printing, something which literally no one uses GIMP for professionally and was a dying need? Meanwhile, Photoshop was demoing an AI object selection tool that could magically select objects without needing to trace them, which came out in 2019. Using GIMP felt like using software from a decade previous.

The last 5 years have been the worst for GIMP. Photoshop has been improving at an astonishing rate. Now it's literally what photo editing looked like in 90's movies - you just open an image, click "select object" and it perfectly selects it, and lets you move/drag/add elements with AI, etc. You can do edits in seconds now that used to take hours, and the results are really good.

None of this is a complaint about GIMP or all the people who contributed to it. It's impossible for a few volunteers to complete with infinite money and hundreds of full-time employees. But Photoshop and GIMP are no longer in the same league. And Adobe knows this, which is why it can get away with punitive subscription-only pricing.

It has been a very long time since I tried GIMP (>15 years) to remember everything I found wanting, but as I recall, GIMP lacks both macros and batch editing, the former letting you record a set of actions to a hotkey so you don't have to repeat them yourself all the time, and the latter letting you apply a set of actions to hundreds or thousands of images at once. I would literally have to spend hundreds of hours to do things in GIMP that can be done with no effort in Photoshop, to the point where it would actually be easier to just program something myself from scratch than it would be to use GIMP, if Photoshop didn't exist.

I see that GIMP has since gotten a UI revamp, but the multiple window UI from the time I used it was also unbearably bad and one of the main things that sticks out in my memory.

As an avid GIMPer for ~12 years now, I hate the UI. It's only fine because I've struggled through it for so long and now I know where and how things are.

But it's really poorly designed and outdated. I completely understand and sympathize with anyone trying to use GIMP for the first time.

Correct, I prefer inkscape and inkpot projects. Don’t use gimp.