I'm not an expert, so take this with however many grains of salt as you wish.
This is a Gateway Sports cover, and they have a good reputation for getting authentic autographs on their envelopes. This is just me - Klrdds' advice to do your research first before buying is spot on - but I would not hesitate to trust the authenticity of an autograph on a Gateway Sports cover.
Here's an unusual Wayne Gretzky Gateway cover from 1982, where "The Great One" mistakenly signed
inside the gold box (where the "silk" cachet image is supposed to be). Below the image is the relevant part of a 1994 flyer from dealer Steve Ciniglio explaining the background of the piece.
