Taking a cursory look I can see somethings you can learn or ameliorate. First is to develop a texture library or the ability to create your own textures seeing as you are using the default GIMP ones (Here's a good source to start from). You also may wanna work with muted or less saturated tones for I find they yield a better overall aesthetic. Second thing is to learn then function of layers and layer masks as these allow for powerful editing (Here's jfrazierjr Tutorial on layer masks). Also you should learn to set a B&W outline as a start point and use that as a base layer to build from, I find the more I have outlined the easier it is to develop a map. When using the Pattern -> Grid filter make sure the offset is set to 0 for both x and y axis so that your grid is centered. The scales that are most common for encounter styled maps is 50 px per square or 100 px per square choose what you prefer and work from there. Take advantage of the internal grid in GIMP and it's snap capabilities to ensure clean lines and ease of manipulation. I leave you with this and wish you goood luck and welcome to the CG.