The #1 way to improve your gut health.
Stop eating foods and drinks that weren't designed to be eaten.
Firstly, the ingredients on the back of a food label should be the ingredients that you intend on eating.
For example, if a snack bar contains oats, dates, cocoa, etc. all of these you would be happy to eat.
When the list start moving into xylitol, sorbitol, bulking agents, monosodium glutamate, sodium nitrite, guar gum, carrageenan, sodium benzoate... and other crap that sounds like it should be in a chemistry experiment, consume it in moderation.
Secondly, become aware of foods that haven't been prepared in a way that is designed for you to eat.
Highly processed foods, bread that you can roll back into dough, burgers that won't go off for 6 months, bars that make your throat close up as soon as they hit your mouth... again, practice moderation.
Finally, drinks. Consume drinks in their natural form where possible: Water, tea, coffee, fresh juice or smoothies, coconut water, you get the picture.
There is likely nothing worse for your gut health than fizzy drinks. Take a step back and look at the liquid in a clear glass and ask yourself, "Is this designed to be consumed" and and you will find all the answers you need :))
To re-itorate, balance and moderation are everything - I myself won't shy away from a processes meal or snack, alcohol, or a bar that I don't even want to look at the ingredients of.
That's okay, just don't integrate them into a daily routine. Stick to a foundation of natural foods in the form we are designed to be eaten in.