I don't like Agassis god-like reference, but indeed the Grand Slam is the ultimate tennis achievement. The surface argument, that Laver could win only on grass, isn't valid. Laver won all four big pro events in 1967 on grass and indoor hardcourts, too. In 1962 he won 9 clay events among 22 titles, in 1969 the bunch of his 18 titles were won on hardcourt and indoor carpet. To the contrary, Laver would possibly have participated and won more majors, if majors were played on hard courts then. Laver was not particularly a big server, he had a hard first serve down the line, but relied more on spin and pace change. He liked true footing for his fast return game and offensive-minded net game. The bad footing on grass at Forest Hills and partly on Australian soil was poison for his swift style of play. As Lendl said, the 5 years interruption in prime years is a factor to be considered, if one refers to numbers of majors won. Gonzales and Rosewall in particular were even more victims of the pro system then. If you take away the last 5 years from Federer's resume, what would be left.
If all pro would be eligible all the time, Laver would maybe not have won a Grand Slam in 1962, but that he would won no major before 1964, is imo very unlikely. He would adapted much earlier to the style of Rosewall and Hoad, if he had played them earlier. Since 1964 he was the king of the hill on the pro scene. It is to be said also, that in the early 70s, promotional struggles prevented contract pro players like Laver, to play at major events, too. So the real open era doesn't begin in 1968, but more in 1974.
Laver is of course right, that all cross eras comparisons are at last futile. Federer's achievement is certainly great in the context of his time. If he had won RG in 2006 or 2007 over his Roadblock Nadal and had gone on to win the real Grand Slam, it would be substantially greater. His RG win puts him possibly over Sampras, because here we have two quite contemporary players. On the other hand, Pete doesn't need to hide himself behind Roger, because his best record - the 6 year Nr. 1 status at years end - will never be broken by Federer. That he had a bit luck in 1995 and 1998, is given, as is Roger's fortune, that in a sudden twist of fate, all his black horses were removed from his path at RG. But credit to both, that they took their opportunities.