Your question bothers me a little, as a Wiccan. Wicca is not goth, and not about being gothic or dark.
Wicca is a religion... Ask yourself:
Are you Wiccan?
Is your fiance Wiccan?
Are you being married by Wiccan clergy?
If not, then no, your wedding is not Wiccan, period. And if it's not Wiccan, you probably should stop calling it Wiccan. You can have all the pentagrams and black that you want but that doesn't make it Wiccan. Wiccans aren't the only ones to use pentagrams in their religion, and certainly are not goth nor is it traditional to wear black at our weddings, so to call it a "Wiccan wedding" just for that is soooooo beyond disrespectful to Wiccans and soooooooo untrue.
If you want to wear a black dress... shop for one. Any dress can be your wedding dress... you don't have to use an actual bridal gown.
Consider your budget... for a real bridal gown made in black, it will have to be custom made and cost a bundle probably.
A gothic wedding gown costume would probably be better, a quality one would still probably cost several hundred dollars.
Alternately, go buy an old black prom dress... or any black dress you like, and use it. Or buy one you like and have it altered a bit. Or take the time to learn to sew so you can make your own and save a bundle.
So much depends on budget and depends on how formal or casual your wedding is going to be.