GI fellowship is competitive due to the limited number of available spots. I don't know how it would compare to derm/plastics, since they are looking at completely different things. Grades/board scores don't seem to carry much importance at all (not even required at many programs). I don't think that a chief year is absolutely required, but it can certainly help. There are many GI fellows that did not do a chief year. From what I have seen, the most important factors are letters and research (presentation/publication). As an extension of that, where you did your med school/residency (the latter obviously more important than the former) and who you know (i.e. big-name attendings in the field) also play a large and important role. In fact, a strong endorsement from a big-name attending alone can often land the candidate a spot.
In short, not having a chief year is not going to hurt you. Having a poster or oral presentation at DDW is definitely a good start.