My exact position about a month ago. As I've learned now, the GECK is a piece of brown poo when it comes to various tasks involved in creating landscape and terrain. The hardest tasks involved in making beautiful alluring outdoors with the GECK and in general would prolly be these things:
1)Creating a good heightmap
2)Painting and shading the texture from the textures palate
3)Making sure your colors, textures, etc. work well under the various weather conditions and light/day. If your ambient light is too high on a snow covered area, the sunlight during the day might blow out all ground detail.
4)Takes along time, very repetitive tasks. A lot of polishing.
It is seriously an art and a science. But more of an art. If you want tutorials on the theory of landscaping in video games, just watch any tutorial series and you might learn something general. Most books on game level design have a chapter on general landscaping theory including do's and dont's. I could hardly find anything on using the GECK landscaping utility. There's some very applicable info/tutorials on the TES wiki http://cs.elderscrolls.com/index.php/Basic_Landscaping_Tutorial. I learned mostly from this website, the Construction Set is similar enough to GECK. Also looking at the vanilla landscape, and then manually recreating it myself. It sounds stupid, but it was helpful to just first try to emulate other more skilled ppl's work.
I suppose that at the end of the day, its harder then easier, but like any other thing in life, the more talent you have the more efficiently and enjoyably you can work.