Please tell me how digestive issues are related to white coating on the tongue? Is it acid reflux/gastritis. Or excess gas in stomach? Bacteria in gut?

It's complicated. It can happen with oral yeast infections, certain digestive issues, and certain viral infections or vitamin deficiencies. It is not generally actually caused by either acid, gas, or bacteria, but by changes in the papillae, or taste buds, in reaction to something happening right there or in the body. If your tongue is not SORE it may not be very important. If it is, you might want to see someone.