Question

why gravity doesnt pulls down a flying balloon?

Answer 1

because as we move higher in the atmosphere it density decreases and as a result ballon which have helium or any other light gases displaces the air at that place similar to that of bottle in water and hence it goes up

Answer 2

$\huge \mathfrak \blue{answer}$

The greater the altitude, the lower the density of the atmosphere. If a light balloon does not change in any way,it will eventually reach a height where the buoyancy related to the air it displaces no longer exceeds the pull of gravity.

The balloons stops rising. As balloons rise, air pressure around them diminishes.