If you're trying to decipher some hidden definition of Glengarry Glen Ross, you'll be looking for a long time—the name just comes from the properties the guys are trying to sell (Glengarry Highlands and Glen Ross Farms). However, critics have referred to the title in such ways as "beguiling melodic" (Source)
There is a musical quality to the title that belies the harshness of some of the language of the play. At the end of the day, though, it comes from two properties, reiterating that it's all about selling in the end.