I would say it is, in the sense that you could write it as "Seeing you is nice", so that "seeing you" is a noun phrase. Am I wrong in thinking this?
Would the same principle apply to:
Thank you for having us.
Well thanks for staying.
It's lovely seeing you again
Thanks so much for coming.
Finally what time are these phrases referring to. I'm planning to teach them as referring to a past event that is still ongoing at the time of speaking.