From Julian's Live auctions, this is one of four hairpieces custom-made for Gene Kelly. "Studio wigs at the time were custom-made and fitted to measure by the star's head and hairline."
Yes, Gene Kelly wore a hairpiece, actually lots of them. Before you gasp, you should know that many of your classical Hollywood favorites also wore rugs:
Classical Hollywood stars wore toupees both on- and offscreen. Sometimes offscreen, they wore no hairpiece at all. For example, here's Gene Kelly without his partial hairpiece hobnobbing with Grace Kelly at the Cannes Film Festival. And here he is again, sans toupee, enjoying a cigarette.
Hollywood hairpieces aren’t just things of the past though. Several current movie and TV personalities also don toupees both on- and offscreen. See, for instance,
At any rate, as this newspaper article suggests, Gene Kelly’s hairpieces must’ve been rather tough to withstand all that dancing and spinning. And you know what, most of his wigs also looked good. They’d better; Kelly's “hair additions” were presumably written into his MGM contract.