You say nothing about how much gas had already leaked out. Or is this assumed to be zero ?
You have that gas is leaking out with , so lets say the time variable is in minutes.
Call the amount of gas leaked out. Then:
Set if none had already out.
The gas forms a cylinder on the water.
This gives us: , since we are using SI-units here, that is calling 1mm = 0.001m .
To find the radius as a function of the amount of gas, interchange V for