Part of the fibrovascular system that transports soluble organic nutrients like sugars to where they're needed.
Regarding the gastrointestinal part - ah, you must have eaten shrimps... (There's probably a reason it's colloquially referred to as the "bananus", short for banana's anus. No idea if there's a scientific name.)
Here's a tip: Do not imagine the act of its removal as "deveining [wikipedia.org]" your banana. (you're welcome) Aren't euphemisms wonderful?