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).
The first one is definitely not a gerund it is a noun at least as i would initially interpret it,
the second one could go either way depending upon intent. Native speakers would probably term it as a gerund as if meeting acted as a verb meaning to convene it could also pose as noun as in a board meeting the noun being the body of persons that constitute the board but in that case it would likely be said "it is our board meeting."