What must be done by one who performs ablution and feels gas (like bubbles) escaping without sound or smell, and repeats ablution many times without success, while being certain of the gas escaping despite her intrusive thoughts?

It appears that what you are experiencing with the emission of gas is a form of waswasah (whispers/temptations from Satan), so do not pay attention to it. Perform ablution once, and do not give any thought to what happens afterward unless you are absolutely certain. If it is not waswasah and the emission of gas is continuous throughout the prayer time, then you are afflicted with chronic flatulence, and your ruling is that of someone excused. In this case, perform ablution for each prayer after its time has begun, and pray both the obligatory and supererogatory prayers with that ablution; whatever is emitted will not harm you. However, if you find a time that is sufficient for purification and prayer, then wait until that time, perform ablution, and pray with a valid purification.