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.