My Gap/Sand/Lob are a set (50*-55*-60*) and as mentioned in another thread have different bounce types. I think these types of sets are probably best for your average golfer as they can cover lots of different shot types without needing to put a whole lot of thought into it. Beyond that, it’s really about feel and preference and making sure that you don’t have any glaring distance gaps that could negatively impact you game (+1 for a good fitting here).
On the subject of sets (especially GI and distance irons) now including gap/approach wedges, we could easily wind up in a long debate about loft jacking. My personal opinion in short is that, yes, there is marketing to it where they want to claim longer irons. There is a legitimate performance piece behind it as well where they can get the height and spin characteristics of traditionally higher numbered irons in lower lofts (and more distance) through technology. Since most amateurs tend to miss short, this does provide legitimate value. Additionally, since a lot of higher handicap players shy away from the low irons anyways (having been replaced with hybrids), starting the iron set at a 5 or even a 6 instead of a 3 or 4 makes a lot of sense.