What Gilles is referring to is that a batch of 1979 OPC Gretzky counterfeits that were circulating a few years ago had an obvious difference of missing a printing error where a white dot on the shoulder. This card appears to be missing that, too. [Only OPC: genuine Topps cards don't have the dot.]
However, I have never seen a report from a grading company about the missing dot being a definitive indication of a counterfeit. (That's not to say there isn't one, just that I haven't seen it.) There were supposedly two printings, and some believe that the second printing doesn't have the dot.
There was also a counterfeit circulating that had red and blue specs to the left of the skate as opposed to just blue specs.
All of this is quite depressing for the future of the hobby (and for the value of our genuine cards). Unfortunately, the technology used for counterfeiting is getting better and better. At the moment it's barely possible for a lay person to tell a good fake from a genuine card.
I worked for Christie's Auction House for years and years. Although we didn't deal in Sports Cards, we did see a lot of counterfeit Photographic Prints come through. A great fake is almost completely undetectable, even to a professional, but costs thousands of dollars to produce. However, the Chinese counterfeiters are getting so good at cranking these out that the prices are coming down fast.
Plus there are so many mid-range counterfeits in circulation already. I don't know if you read that FBI report from Operation Foul Ball. But one of the guys they caught in that big ring estimated that over 50% of non-card sports memorabilia (balls, bats, jerseys, helmets) on the market today is counterfeit. Cards won't be much better a decade from now.
Here are some useful resources to educate yourself about fakes and altered cards (which we haven't even discussed)...