What you eat feeds something. Some fibres are not digested in the small intestine and reach the large bowel, where microbes can use them. Different fibres support different microbes and may lead to different fermentation products.
Variety matters. Plant foods such as vegetables, fruit, legumes, nuts, seeds and whole grains can contribute fibres and other compounds. A gradual change is often easier to tolerate than a sudden increase, especially for people who already experience gut symptoms.
Fermentation is not automatically a problem. It is part of normal microbial activity; the location, amount, individual tolerance and overall context matter.