I'm also pretty new, but I've done a fair bit of learning recently so how I went about it is rather fresh in my mind, and my preferred tool for digital artsy things in general has been GIMP all the way. So I'll share my thoughts!

I tend to learn best by doing when it comes to creative ventures. When it comes to large-scale maps I still mostly follow tutorials, but various techniques used therein have become more and more familiar to me and I'm starting to wander off the beaten path a little bit more here and there to try and get the sort of effects I want. And overall I would say I have been gaining some real understanding of the reasoning behind some of the methods used. So if the creative juices keep flowing you might not worry too much about haring close to the tutorials for a little while - try multiple different methods, in different styles. A lot of my learning has been in the process of working through two major tutorials on this site (one of which might be the one you're referring to, were I to guess):

Thread for hand-drawn style maps in GIMP by Gidde
Thread for regional maps in GIMP by RobA

While they are tutorials you can follow step-by-step to a result, as you get more familiar with those steps you can start to fiddle with the pieces more and more comfortably (at least that was my experience). For example, one map I was following Gidde's hand-drawn map tutorial to make was of an area that had a swampy wetland region and a red-clay mesa/plateau kind of region that I wanted to give appropriate colors to. Gidde's tutorial only discusses coloring explicitly for seas/lakes, rivers, mountains, hills and forests, so I needed to experiment. The tutorial used three different general methods for those colors (the seas/lakes and rivers, the mountains and hills, and the forests), using different base tools, filters, and layer blend modes to achieve the effects, so first I tried to make the colors for my additional regions using those three basic methods with different color values but also toyed around with other blend modes, for instance, to try to get an interesting effect. All of that served as a learning experience that gave me more tools to consider applying in future works.

Every thing I did gave me a little bit more knowledge with what this and then that small piece of GIMP's broad tools and capabilities did, although I did a bunch of other small things, like trying to throw together impromptu battle maps, which also gave me opportunities to use what I'd learned so far as well as reasons to attempt something new and see what worked and what didn't. If you learn in a similar experiential/experimental manner then just trying to make lots of different maps can slowly broaden your actual understanding - even if you just follow a bunch of tutorials, the commonalities and differences between them may start to illuminate understanding. It also helps to be unafraid to look up 'how do I do X' if there's some other thing X you want that a tutorial doesn't cover for what you're working on. For example, referring to Gidde's tutorial again, on one of my early maps I wanted to use brushes for mountains but wanted to draw the mountains myself rather than using a supplied brush, so I looked up how to create those multi-element randomized brushes in GIMP and learned how to make my own brushes for placing mountains, hills and trees.

That said, I'm sure there are more 'fundamental' resources out there - one I might recommend that I noticed here was a thread on layer masks; I've found layer masks to be a super useful tool in general for trying to make all manner of things look nice in GIMP, enough that I'd probably consider them of fundamental importance, but they weren't an immediately obvious feature to me at all when I first started out.

Layer Masks explanations by jfrazierjr

Finally, I guess to sum up what I've said above in brief: don't overlook the value of just gaining experience. You'll probably pick up more than you realize by just doing lots of different things - even if at first those lots of different things are just following different tutorials to make one thing or another in GIMP. One tutorial might have you use the 'stroke selection' utility, which is a tool you can do a lot with in many settings; another might showcase a few useful properties of certain blend modes. If you have things you want to make, that can give you direction, and if you have clear ideas that direction may lead off the beaten path in a few places while following a tutorial that otherwise guides you through. Those moments are great times to experiment because it can be a small, contained case of 'how do I get this effect that I want using these tools' without being so broad you get lost in the weeds while fiddling with different ideas of how to accomplish it.

I hope that is somewhat helpful - I know I was a bit short on having actual fundamentals tutorials on hand to share, although I know they're out there (here and elsewhere) if you go looking - I just wanted to point out that there's still a lot to be gained from the step-by-step tutorials, as I tend to find the 'start from the very basics' expositions to not be very compelling as compared to something that gives more opportunity to use what knowledge I find.