Is it necessary to wait until flatulence and gas subside to perform Fajr prayer, given the fear of missing its time?

Merely feeling bloating and gas in the abdomen, or suspecting their emission, does not nullify Wudu' (ablution). Rather, it is nullified only by certainty of their emission. It is recommended to wait for those who have the capacity to pray without anything being emitted. However, if waiting leads to the Salat (prayer) time expiring, then one must perform Wudu' and pray in the current state. One should not wait for the gas to completely cease if it takes up the entire prayer time. What you are experiencing is a medical condition, and it is advisable to consult a doctor.