Well, first things first. If you don't know, Gulzar's real name is Sampoorna Singh. Gulzar was the name he adds as a poet, making it Sampoorna Singh 'Gulzar'.
Gulzar actually is a word that is used for a Garden or something similar when it is full of flowers. Gulzar is a Persian word with its origin being Gul, which literally means rose, or generalizing you can say a flower.
Thus, Gulzar is a state of a Garden that is blossoming, that has a lot of flowers there.
Oh, and by the way, Sampoorna means complete.
I hope I have been able to get to you the meaning of the great lyricist's name.