I tried to add some sort of shading to the upper layers, not finished yet but here we go anyway. I'm no pixel artist, but I think it looks pretty alright and adds some kind of depth. What do you think?
I don't know about existing tiles, but yes you can use gimp.
Use layers and brush effects with burn, overlay, multiply; and leaf brushes and/or textures.
Try looking in the resource section of places like deviantart, there should be some free to use brushes you can use to mimic the wear and tear.
When you can draw inspiration from real world references. There are entire sites dedicated to exploring abandoned structures and overgrown buildings that should give you some ideas.
Add grit along the open surfaces, particularly in corners and along cracks, where dirt would settle and stuff would grow. if plants are growing they tend to drop leaves which form mulch, panels get pushed out by expanding roots, glass is almost ever clean with dust and pollen, plastics may discolor, etc.
Happy Wednesday y'all. Posting a screenshot with some updated assets. Tackling the pavement will be coming soon. I'm starting to gather a fair amount of assets for general map clutter, but I need to break up the green pavement next and give it some character. Now the solid green is just in your face. Still not much for shading yet either. Hope everybody has a good Wednesday.
That moment when things actually look like they are coming together. I have posted this building before, but the screenshot was the asset within Pyxel Edit. This screenshot is in the MV editor and everything now tiles appropriately and has some level of shading.