The plugs I used were NGK which cost about £16 for 8 from an apparently (as well as you can tell) reputable seller on ebay, this was the going rate for these spark plugs from lots of sellers, when I changed them it was identical ones I removed, these are standard life plugs, should be changed at 12,000 miles or thereabouts, more expensive, extended life ones are available from NGK and other brands. I have no reason to suspect the plugs I got were counterfeit, I am a former engineer and examined them before fitting, all appeared in order.
GSF are a reputable company, I am sure the plugs they sold you are the genuine standard life plugs, by the way, they last a lot longer than 12000 miles.