GI is a three year fellowship done after completion of a three year internal medicine residency. I am currently doing an elective in GI and I can attest that it can be quite busy as can most areas of medicine. In general however there are less emergent situations that require middle of the night intervention with the obvious exception being significant GI bleeds. The compensation is quite competitive as the field is procedural in nature along with the current screening recomendations for colon cancer.
The fellowship is very competitive currently probably only taking second to cardiology...but even that is open for debate. Placement depends upon superior performance during your medicine residency, this can be assisted by first of being at a well known academic center and secondly becoming involved in research in the GI field, in essence becoming well acquainted with the GI people at you institution making some connections.
Hope this helps and best of luck