Hi there Ver, and welcome to the Guild. As to your question regarding mapping an image to a globe in GIMP, you should be able to map your entire world to a globe. I wrote a tutorial on the matter a while back (you can find a link to it in the "Tutorials" section of my sig). Personally, I use GIMP along with Wilbur (which also can help to either generate realistic terrain, or manipulate a height map you've made in another program, such as GIMP). I have not worked with FT3 myself but I have heard good things about it. Waldronate, another member here, could probably advise you on the relative differences and benefits of FT3 and Wilbur, so I'll leave that to him.

Cheers,
-Arsheesh