I was making a map today (Hit M in editor). I was using a .gmap file rather than a .txt... At first it went normally with the filenames flashing by... but then an error window popped up saying: "List index out of bounds (625)"
You're GMAP is broken, did you play with it?
Open the gmap in the text document, and check the width + height (levels across, levels down and insert them in)..