No. Sippyjug, not to single you out as many others in this thread have made similar claims about GIMP. Your post is just the most recent and someone needs to say no to this misinformation about GIMP. No it can't do all anyone would want.

It can't do what I want and all I want to do is simple stuff like darken a sky -- efficiently. Yes GIMP can do it but not efficiently. GIMP is way too expensive because it's so inefficient. Here's an example. I attached a photo below. It's a straight raw conversion with no additional editing. First job is to darken the sky.

I don't want to just drag a gradient over it because of the tree branches. I want to darken and saturate the sky with little to minor effect on the tree branches and I want to keep the the tops on the trees on the left side looking natural. I also just for reference attached my processed version of the photo so you can see what I want in the sky.

Can GIMP do it? Yes it can, but can it do it easily and efficiently? An emphatic NO! I recorded the steps it takes to just darken the sky using three different apps. Here they are:

Capture One
one mouse click add layer.
one mouse click type value set exp reduction.
one mouse click type value raise saturation.
one mouse click activate luminosity Range.
two mouse clicks adjust luminosity range limits slider.
one mouse click turn off display mask.
two mouse clicks adjust radius & sensitivity sliders
one mouse click OK.
one mouse click activate mask eraser and remove unwanted mask in water and grass -- done.

Photoshop
one mouse click duplicate layer.
one mouse click set blend layer to Multiply.
one mouse click turn off dupe layer.
one mouse click select magic wand.
4 mouse clicks to select the sky.
one mouse click turn back on dupe layer.
one mouse click add layer mask which conforms to selection.
one mouse click to bring up Refine Mask dialog.
4 mouse clicks with slider adjustments to set refine mask parameters.
one mouse click OK.
one mouse click layers palette adjust dupe layer opacity -- done.

GIMP
one mouse click duplicate layer.
one mouse click Colors menu.
one mouse click Desaturate dialog.
one mouse click set parameter to Luminance
one mouse click OK.
one mouse click go to Channels.
one mouse click and drag Green channel to duplicate channel box.
one mouse click back to Layers.
one mouse click select background layer.
one mouse click Select menu Select all.
one mouse click back to Channels.
one right mouse click on dupe channel to bring up sub menu -- select subtract from selection.
one keyboard command Shift Ctrl A to remove selection.
one mouse click go to Layers.
one right mouse click delete the duplicate layer.
one mouse click duplicate background layer.
one right mouse click on dupe layer select add layer mask.
one mouse click in layer mask dialog -- select Mask to Channel.
one mouse click drop menu select green channel copy
one mouse click OK.
one mouse click change blend mode to Multiply.
one mouse click select paint brush.
one mouse click make paint brush big.
one mouse select layer mask.
multiple mouse clicks paint layer mask black everywhere except sky
done.

Notice how much more involved the process is in GIMP. While someone -- assume a GIMP guru is stepping through that I'm finishing the rest of the edit in C1 and I'll be on to another image while the GIMP use is applying another excessively involved and convoluted sequence of steps to try and finish this edit. With serious work to do I get the weekend off and the GIMP guru burns the midnight oil all seven nights of the week.

GIMP in theory could do the job the same way I did it in PS (Affinity can -- if not better) except it can't because it lacks that Refine Mask Edge option in PS (and Affinity) and because in PS you can see what you're doing but in GIMP trying to refine a selection edge is really irritating because you can't get a good look at what you're doing till you do it and then go oops and undo and then redo it and go oops and undo it and then redo it and go oops and then undo it and redo it and then settle because you can't stand it any more. Reading that last sentence is often what it feels like to work in GIMP. Yes GIMP can do it but you don't get the weekend off.

I used GIMP about a decade ago. I switched to Photoshop because it was more capable. There is a difference in the user interface that takes some time to get used to.

I was reluctant to go with the subscription model when Adobe started it but eventually realized that it was cheaper for me than paying for updates as I had been doing.

Any hobby incurs costs. Photography is no exception. $10/month is a low cost part of the hobby for me. Costs less than monthly impulse purchases and provides me with more lasting value than the impulse purchases.

GIMP is the Gnu Image Manipulation Program.

You are not alone in that. I've been a Linux user since 1998, and have found GIMP more than meets my needs. If you are not a pro you likely don't need the color management features that really make PS the choice for many. I've played a bit with layers and masks, and they are capable enough for anything I've tried to do. And the companion application Darktable will probably satisfy your RAW needs, unless you have a pretty new camera (my version doesn't handle CR3 (from the EOS R5) yet, but it should soon).

Recently I made a HUGE error: I got an email message to update my operating system on my Mac to Big Sur. I installed it. Little did I know that I was making my CS6 Photoshop program and others non-functioning. Adobe of course wants everyone to pay a monthly subscription fee to have Photoshop and I am now looking into other options. So far the one I am gravitating to is Gimp. It is similar to Photoshop but no fee. I'm wondering if any of you UHH members have switched over to Gimp and if you have, how do you like it? I've been using Photoshop my whole photographic time and didn't want to have to learn a whole new program. I have downloaded Affinity Photo for the free 90 day trial period but it's not really very user friendly. Thanks for your advice in advance.

You have choice. I run both Windows and Linux on the same computer. It's not hard. I can't think of anything more backward than deciding what application software to run based on the OS. An OS exists to support application software. If it doesn't then it has no reason to exist. I don't care what OS I'm using -- I can't process a photo with an OS.

No matter how good you get using GIMP, you have to do more work for the same result. Biggest problem with GIMP is you often lack visual feedback in process. You have to commit an action to see the result and then evaluate if the result is acceptable -- if not you have to try again. That is so bogus.

The cost argument is silly. I like many here have multiple cameras. That's thousands of $$$. Add in the lenses and how could I possibly complain about my software cost at around $150.00 per year. If I can't afford the software I have no business affording the cameras.

Choosing to use an editor like GIMP increases your requirement for disk storage substantially in order to save your work. That's a hidden cost. GIMP is expensive to use.

For those who don't know I provide useful objective information. A question was asked and should be answered with helpful information. I'm able to provide that.

Do whatever you want. You don't even have to tell anyone. You being happy with what you do doesn't change any facts.

In a raw workflow GIMP is a destructive editor -- Photoshop much less so. That's a fact.

In use processing photographs GIMP is a less capable, less effective and less efficient solution when compared with many commercial products -- especially Photoshop. That's a fact. You can't argue otherwise and folks deserve to be told that if they're trying to make a decision. It may be appropriate for an enthusiast to nonetheless decide to use GIMP. What's inappropriate is for them to make the decision with misinformation.