Aw, when I saw the title, I expected something pretty amazing. Sorry to discount your work, but this isn't amazing. You need the buildings in the background anyway. You should try to mimic the actual map a little better. Sorry to disappoint ya.
Any, you ought to multi-texture. The lil' island in the water should be sandy. You ought to download some of the older Gm_Construct maps to get a better idea on how to manage making this. Older versions will be easier to replicate. A 3 dimensional map is hard to replicate most of the time anyway...
Find the Garrys Mod download site and download: Gm_Construct Legacy for the older map versions...