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.