Yes. The sentence could be rephrased without the it, as "Meeting you was nice." You wouldn't usually say it that way, but thinking of it that way shows that meeting is the subject, was is the verb or predicate, and nice is a predicate adjective. Since meeting is a form of the verb meet, but as the subject of a since is functioning as a noun, it is a gerund rather than a participle (the other verb from that ends in -ing).