An esrog with a gartel is not mentioned in any of the classic poskim. According to Wikipedia:
. . some look for an etrog with a gartel—an hourglass-like strip running around the middle, more commonly found on the Moroccan citron. According to researchers, this gartel indicates when the bearing tree was infected by a certain virus or viroid, which decreases the albedo on the specific spot. These viroids have been around since at least the time of Bar Kokhba (circa 130 CE), based on the fact that archaeologists have unearthed a mosaic dated to that time which depicts an etrog with a gartel. Only the etrog is found to be susceptible to these viroids, proving again that the etrog is genetically pure and has not changed significantly over the centuries.
Also, according to here some consider it more beautiful.