Must prayers be repeated when the emission of gas, which occurs frequently, ceases, given that its cessation is unknown, and what should I do when gas is emitted only during prayer?

If you do not know a specific time when the emission of wind stops, then your ruling is that of one afflicted with perpetual discharge (sahib al-salas). Therefore, perform ablution for every prayer after its time has begun, and pray with this ablution the obligatory prayer and whatever voluntary prayers you wish. Nothing that is emitted from you will harm you.