I like that you added the software and specs. Gimp is rad, I use a version called Gimpshop skinned to look and operate more like Photoshop. Props for drawing with mouse. I've never held a tablet, look at all these cool kids running around with Wacom tablets, kids these days ¯\_(ツ)_/¯
I've never downloaded brushes either. You obviously didn't use any special brushes, I think? It almost doesn't look like a pixel image, gotta zoom in to see the square details. As someone that does not pixel art help me understand, did you draw this by hand first? It's a really good illustration <3