Meaning
The rose and the nightingale; the classic pair of beloved and lover.
Literally: flower and nightingale
How Iqbal uses it
Gul-o-bulbul, rose and nightingale, names the archetypal romance of Persian-Urdu poetry, beauty and the one who sings for it; Iqbal inherits this whole symbolic world even as he pushes its lament toward action and selfhood.
See it in the verse