It is native, here in the S. Jersey pines and quite common. The soil is very acid. It makes a weak, sparse, but always beautiful cover. I am confident it will not crowd out other plants, which is sometimes good and sometimes not what you want. I have found, in contrast, that my Epimediums usually crowd even periwinkle out.
I think that Gaultheria spreads slowly, and the huge plots I find must have been there many years. It is in the forest, where considerable Winter Sun gets to the ground but Summers are in shade. The shade is usually not total, but the result of high trees, with considerable space between.

Bogturtle:
Thanks for the info. I don't think that it'll work for me. Mabe I'll look up epimedium. I wouldn't mind an aggressive cover!

I would caution you, as I stated, just now in another posting, that most of my Epimediums look really bad, this time of year. I string trim them down to make way for beautiful new foliage and the fine little flowers. The only maintenance they require. There may be some sorts that are more evergreen than the kinds I grow.

Never have been successful in posting photos to garden web. Most people seem to do fine. But I maintain a non-commercial blog on gardening that is, I hope, totally inoffensive, and have posted photos of my Epimedium and periwinkle with comments. The ref. to it is on My Page.

I tried it last year and its a very slow grower. I bought mine from "Raintree Nursery" (catalog and online). I buy a lot of stuff from them and its very good quality...but I did "everything right" and pampered the silly stuff all spring and summer. It never really grew much. I missed one watering and it died. I'd think more toward a vinca - much easier (but it can be invasive). Raintree also had a cute little groundcover called a "kinnickinick" - which is more like the wintergreen and has edible berries. It did really well, and it is really cute. And it kept its leaves all winter.

I also grow kinnikinic or Arctostaphylos ova-ursi, the bearberry. Mine is 'Massachusetts'. Amazingly, the rabbits did not leave it alone, but otherwise,it has been a splendid and easy groundcover. Do not fertilize it.
I cannot say if Vaccinium crassifolium would do well in your zone, but the variety'Wells Delight' is beautiful. The rabbits will total this for me, also. I have been wondering about the 'bearberry' cotoneaster. It might need Sun, and the rabbits attack both sorts I have here. What about Galax aphylla? Mine is new and so I cannot say alot about how it does.