What is gynostegium?

Hint: The reproductive part of a flower is divided into calyx, corolla, androecium and gynoecium. In the flowering plants, the flower shows some modification by fusion of its part which helps to attract the insects for pollination.

Complete answer:
Gynostegium is formed by the fusion of stamen with stigma. It is a complex structure where all parts get fused at the stigmatic surface.
The sexual reproductive organ of a flower helps in sexual reproduction by the formation and fusion of haploid gametes helps in the production of a diploid zygote. The flower of the plant consists of calyx, corolla, androecium, gynoecium and thalamus. The fusion of these parts helps to modify the structure and also to attract insects for pollination. Two common types of fusion are postgenital and congenital. Postgenital fusion takes place during flower development in the epidermal parts whereas congenital fusion takes place during differential growth in the meristem part of a flower. In case, congenital fusion does not take place completely then the presence of free organ parts in the common base get divided completely into early and late congenital parts at the base. In case, the fusion of calyx and corolla takes place then the perianth is formed above the thalamus part of the plant. Depending upon the fusion of flower parts the formation of fruits takes place.

Note:
Gynostegium is common in the milkweed family, like Asclepiadaceae. In this family, the gynostegium is commonly colourful and with calyx and corolla, they form a distinctive structure that attracts insects for pollination.