As a 66 mag, the GP range wasn't out yet so this was simply a marketing ploy for Lambretta, making the most of trends in the market. Interesting to note that this was in a British publication and that Lambretta Concessionaires were so fixed on the 'GP' theme for promotion that they then pushed it to the point that the English Speaking world were presented with the next GP range of models, whereas the rest of the world received the Dl range (125 catching up a little later).
This supports the idea that Lambretta Concessionaires actually wanted to push the GP idea even before the range that we know as the GP was even available. It also shows the power that Lambretta Concessionaires had with Innocenti, which in turn supports the stories of Concessionaires nearly persuading Innocenti to put the twin into production.
Adam