Can smoking weed cause periodontal (gum) disease?

Not well known. I do not know of any well-established correlation between smoking marijuana and incidence of periodontal disease.
Not the cause. But if you have some inflammation in the gums, then nicotine makes it worse. Not too sure about marijuana, but it probably doesn't help any. When smoking weed, then you probably don't think about taking good care of your teeth.
No studies. There are no studies of which I am aware that addresses a correlation. However, it seems that moderation is usually the best course.