Very nice! I'd suggest either going with a stronger grid than you have or removing it altogether. As it stands it's just a bit too faint to be useful. Also, I'd suggest that you don't have the walls casting a shadow onto the top of the doors - it makes it look like the doors are painted on to the floor rather than rising up from the floor. Note that most VTTs can lay down a grid so it is not really necessary to have a grid at all if this is meant for VTT play.
It might also be nice to have the water as a transparent fill, rather than the opaque fill you've got there. You'd need to extend the floor texture under the area the water covers (if you haven't already). Then see what it looks like if you switch the water layer mode to hard light or soft light. Try duplicating the water layer and having one as a hard or soft light layer, and the other as a normal layer with low opacity.
That's a very usable map, and a great first shot with Gimp. Is there anything you would like to do differently about this map?