What exactly does bicarbonate (sodium bicarbonate) do when it is introduced into the gi tract?

Acid neutralization: I'm assuming you're asking about stomach acid, which must be buffered before proper digestion of food can take place in the small bowel. The pancreas assists with both functions--by production of bicarbonate (sodium bicarbonate) that neutralizes stomach acid in the material that enters the small bowel, as well as production of enzymes that work to digest starches and proteins in that ingested food.