Compare with Sketch, which has taken scores of users from Illustrator.
I know several professional designers who only (or mostly) use free software.
They do this as a matter of choice.
And there are many more professional designers, who use free software for parts of their workflow.
They do it, for many different reasons, but mostly because it just happens to fit their needs.
Some of them use Gimp.
If you would have read the article linked above, you would know that it's mostly about a professional designer (Aryeom) who is using Gimp for her work on an animation film.
Personally, I still believe that you're question would have deserved only a "42" type of answer.
"Professional designers" is such a vast term...
But, hey I'm so generous today :-)
P.S.: I don't want to get into a discussion on who is a professional designer and who is not, so I avoid putting links to the ones I know are using only/mostly free software...
But here are the names of the first ones that comes to mind: Cédric, Elisa, Vlada, Camille, David, ...
I wasn't able to do it programatically using Adobe tools. FOSS on Linux did that for me.
It's the "crooked" licensing scheme.
PS allows moving around a few sliders while looking at the screen to see how things change and nail it when the results look visually good. I bet that is critical for most PS users. It is for me. I paid for an (educational) PS license just so I can edit my photos before I print them and hang them on a wall.
One example: doing levels. One slider move back and forth to see which areas are overblown in each channel. One extra keypress to switch between final result and overblown only. That, to me, is so powerful. I can decide on the fly, "well, I lost detail in yellow channel here, but extra detail there is clearly worth it".
I am sure GIMP is great in many ways, but I guess their target user group is different. Their interface just does not work well for me (and I did try), they focus at different things, etc.
I hate Adobe (both their licensing and privacy policies), but CS is just so good. It is one of the three programs for which I still have a Windows partition on my computer (I do most of my work in Linux).
Gimp is just full of little annoyances that interrupt your flow and make it difficult to work efficiently.
Recently I had to make some end-credit cards for a short film - ie. white writing on black backgrounds. Gimp managed to make even that painful. Things like, placing an immovable dialogue over the handles of my text box so I can't get at them. If I try to replace text, it reverts to its arbitrary choice of font / size so I have to type new text in the middle of my existing text, then delete the original. No "duplicate layer" shortcut. Just gratuious annoyances that are good for nobody, and bad for everybody.
Edit: I have a feeling you might be a Gimp dev, in which case I should be more grateful for the work you do, which I still use and benefit from even though the UI frustrates the hell out of me.
Edit2: I just found the Duplicate Layer shortcut. Not sure how I missed it before.
But as for 42, this is unlikely the answer you wanted to hear.
Why should we care? Why not judge it on its own merits?
This reminds me of how a lot of people used to respond to the notion of using GNU/Linux distributions as free operating systems suitable for day-to-day desktop use. Sure, I get why not everyone might want to use it (nobody is asking you to), but why does every bit of positive news or cool achievement have to get accompanied by dozens of comments by authors directly comparing it to a commercially available alternative, stating that they'd use it if only it did this and if only it looked more like that?
I love how we as a global community have such a wealth of software tools available; to use for free (legally!), with a philosophy that explicitly encourages sharing and enables anyone with the know-how to contribute. Some of these tools have been with us for decades, such as Gimp, and each release brings new features and improvements. Instead of celebrating its strengths, software like Gimp gets scrutinized by holding it to an arbitrary standard it can never meet (the full feature set of Adobe Photoshop), simply because the goal posts are forever moving.
I do not intend to berate anyone genuinely asking about how to migrate to free tools like Blender, Gimp, and Inkscape — questions where the comparison of features is warranted — but a lot of comments appear to dismiss tools like Gimp out of hand simply because they aren't carbon copies of the most popular proprietary alternatives.
With operating systems we've entered the next phase, where using a free software operating system isn't cause for raised eyebrows, and where people are actually asking the reverse question: why can't Microsoft Windows or Apple Mac OS X do this thing I can do easily on GNU/Linux? Hopefully we can reach that level of acceptance with other free software as well.
This is true especially for Gimp. I imagine there are many people like me, who work on Linux and already use Gimp for their own stuff, but since Linux + Gimp is usually the toolset of a technical person, most of the time we work with graphical tools it is not our own work, but something a designer created with OSX + Photoshop.
So please, Gimp, create a way for me to fund everything related to better .psd support. On everything else, Gimp can and should find its own way. I like Gimp more anyway due to decades (wtf, I am old) of usage.
Another thing of value for me is that open source can not die for as long as there's demand. This makes it a safer bet for long term investments. So GIMP may never be on par with Photoshop and the Linux desktop may never happen for ordinary people, but you can bet they'll still be around and probably maintained and developed in 50 years from now, because they aren't subject to the same market forces as proprietary stuff.
Hence the comparison is often unfair. For me GIMP gets the job done, for my needs it is perfectly suitable, works on all 3 major desktop operating systems, therefore it has an unbeatable price tag and license.
In this case, it's because Gimp is frequently claimed to be a Photoshop alternative. For most Photoshop users, that isn't really true, and it's very reasonable to point out that it isn't true.
I gave up Windows over a decade ago, and I gave up doing any serious graphics work at the same time, because the best available replacement for Photoshop was Gimp. I still don't think Gimp's up to the standard of the last PS I used (7?).
I'm happy that Gimp exists and I'm excited about the GEGL stuff. I wish people would stop pretending it's an alternative to Photoshop. If you can replace your Photoshop usage with Gimp usage, Photoshop probably wasn't the right program for you in the first place.
I also deal with large format printers. These are built in the thousands or tens of thousands and don't get the software love that they would if produced on a larger scale. RIP's tend to be finicky and are designed to play nice with the adobe suite, so most stick with those tools.
TL;DR ADobe has it pretty much sewn up
While I've been a Photoshop user since v5, I haven't always had it installed. There are even times I've done professional work on Linux where Photoshop doesn't run.
Currently I have Photoshop, but make use of the Gimp as well.
When there are so many free tools like there that can do a pro job - the question isn't "Gimp or Photoshop" but "Gimp, and Photoshop?" I could replace Illustrator with Inkscape and still get my job done, but I have bought Illustrator so I get my money's worth out of it. But that doesn't mean I not longer also have Inkscape. Of course I use Inkscape where it's better than Illustrator :D
Sure it will be more inconvenient, but it's doable.
Give people better tools and it's hard to go back. Can you imagine writing a large piece of software without autocomplete or syntax highlighting these days?