GI lifestyle will vary in terms of where you go for fellowship and where you end up for practice.
Fellowship -- For the most part, your days at work are busy between endoscopy, consults, and clinic. After hours calls are primarily related to bleeders and probably the occasional acute hepatitis. Remember, 80% of bleeding resolves on its own, so for the most part you can probably help manage these patients over the phone if you have residents you trust on the other end. However, among your goals in fellowship is to get your hands on the scope as much as possible and to develop your own level of comfort with what can wait and what can't, so more often than not, at least during the early part of your training, you'd go in. That's probably the proper thing to do anyway. From what I've seen at my own institution, first-year fellows in July thru September come in all the time, then as the year goes on and they develop their own acumen and comfort level, they will come in for selected cases only. Otherwise, keep the pt. NPO +/- bowel prep and we'll scope them in the morning. Otherwise, your procedures are elective or semi-urgent, and can be done during fairly reasonable hours. I guess the exception is if you are the fellow on biliary and someone needs a stone taken out -- you want the ERCP, so you go in. You get the idea.
For most fields (GI, Cards, etc.) I don't think that fellowship is a true representation of what life in practice will be like, in so far as in fellowship your responsibilities are somewhat segmented by rotation. In the real world, you are the consult fellow, the biliary fellow, the liver fellow and the endoscopist all at the same time. Same as in Cards fellowship -- you are on consult, cath, ECHO, all as separate rotations. Plus, you have residents around to help do all the grunt work.
As for the real world, I think the type of lifestyle you'll have will depend on your practice situation, e.g. # of partners, how long you've been with the group, how many hospitals the group covers, etc. From what I've observed, private practice GI docs are crazy busy during the time they are awake, but probably not up as much at night as say people in Cards are with angina and arrhythmia. However, the realities of being a referral based specialty can make cases that could wait till the morning happen sooner rather than later, e.g. scope the pt. now to please the referring physician, rather than putting it off till the AM when your schedule is full anyway. One of my co-residents' Dad is a private practice GI doc who has been in practice a long time, one of the senior partners in his group. She tells me he hasn't gone in at night in a LONG time, but that he is on the phone quite a bit at night giving orders, etc. for bleeders (for example). But his days are jam-packed. He is probably more typical of GI docs who have been in the game a long time. When you're low man on the totem poll, you have a practice to build, but as you get more senior, you can choose what you do and don't want to do.
Hope this helps.