Hey there...and Welcome to the Guild! It's not so very terrible.
The first thing that pops out at me is that your building bevel would look better if it was just on the buildings (red shapes) instead of on the overall building blocks, which include some alleyways. Not sure how that has happened in your map but maybe your alleyways aren't fully erased? Also a bit of a harder "crease" on the bevel might crisp them up a bit. I'm not a GIMP expert so I'm not sure which buttons to push or I'd give something specific. If you get some of this worked out and are feeling adventurous you might think about some color variations in the roofs.
Some ground and water texture would help out.
Looks like you've tried to use some "found" floorplans for the cathedral, keep, and high castle. They're cool but not blending with the style you've put together. I'd suggest maybe using those as references and creating them from scratch in your map. Also, these are showing the building interiors, which is breaking the impression of a bird's eye view of your town.
The forests look pretty good but get a little "wiggly" because of the broad fuzzy band that's around them. Is that a drop shadow?
The mountains are very abrupt, but the shapes and coloring are nice. Ideally you'd like to maybe have some rock surface poking out from at least a little vegetation. Maybe you can put a layer over them and paint in some...or drop them below your grass layer and erase out a bit.
I like your dock shapes...pretty cool. Maybe distinguish them in color a bit from the sandy road color?
The rivers look pretty good and the coastline is nice, especially in the lower right. Rounding off a couple of those little islands might be nice...as they are the first impression is that of a small river that runs coast to coast rather than an island.
Don't think I said so already but I like the overall shape and flow of your town's layout. The mix of building shapes and sizes is good too!
Have some rep for getting rolling!