The intelligent scissors are in the toolbox on the left of the program's screen. Looks like scissors with a line going from it. Is just like the path. Start at a spot and add a node every so often as you go around. When you have finished you need to finish at the first node then hit enter to make the selection active.Or there are dozens of other ways to make your selection. I don't normally use scissors select but thought I would give it a try this time and it worked well
By the way it could be as simple as selecting your 'sky' with the fuzzy select and then put a gradient in it but as I said the lines are not visible around the top centres of the buildings so the gradient will go in there as well. If Gimp will not do what you want go back to Photoshop
