Updated: May 27, 2020
For those of you feeling confused by the namedropping exercise in the title, things are simpler than they read. GIMP is a pretty reasonable, feature-rich image manipulation program (hence its recursive name). It comes with a range of image filters, allowing you to make changes to your photos and pictures, like blur, glow, noise, and many others. But sometimes, even this nice repertoire isn't enough.
To that end, you want something like G'MIC, an image processing framework that comes with loads of extra tools that you can use. For example, you can do color, light and pattern changes, add artistic touches, repair inconsistencies in pictures, render shapes, and then some. The full list of G'MIC features is way too long, so I must resort to: trust me, I'm an engineer. Actually, I'm not, I'm a physicist, so my word is doubly good here. Anyway, in Linux, setting up G'MIC is trivial. In Windows, less so. That's why we're here.
Download and extract G'MIC
What you want to do in Windows is: download the zip archive (from the official site linked above). Now, you can also download the installer and let it do everything for you, but it is better to use the archive, because it gives you a better understanding of what takes place behind the scenes, you have full control, and you can use it with the portable version of GIMP, as well.
Extract the archive anywhere you like. Then launch GIMP, go to Edit > Preferences. Then, in the sidebar, navigate to Folders > Plug-ins, and in the right pane, click on the + icon to add a folder. By default, it will have the standard plug-ins, available as part of the program installation, and any additional extras you may have saved for your user. There could be other custom locations.
We want to add the folder extracted from the GMIC zip archive. It's named gmic_gimp_qt by default, but the name could be different. Locate the folder and add it to the list. Then, restart GIMP (close and open again). On next launch, G'MIC will be available under Filters as G'MIC-Qt.
Now, the plugin will be grayed out if you have now image loaded in the main interface, so don't think something is actually wrong. Most filters and options are not available without some file to work on.
Curiously, the plugin loads with the dark theme as a hard-coded option - it does not inherit the theme from GIMP. But you have the option to change this right away, and work with a nice, airy, light interface the way the Ancient Spirits of the Software intended.
And that's it. Simple, straightforward - and yet, with a snag that could trip people who are not used to the somewhat convoluted nature of plugin configuration in software. GIMP is primarily a Linux tool, so the flow mostly reflects the modular nature of that operating system. In most Linux distributions, you can use the built-in package manager to add the G'MIC package, and Bob's your uncle, so this setup is not really required.
Well, we're done, and now you have an awesome filter pack to help you make your photos even more majestic than they are. Provided you have an inkling of artistic skill and decent camera, that is. I hope you appreciate the non-standard method by which we did this, as it helps you understand how you can manage other GIMP plugins in general, which ought to give you a sense of flexibility and a peace of mind. Take care.