According to all of the Gold medalists, and even Fed. And according to all of the worldwide sporting audience.
I think you may have lost a bit in interpretation about what Nadal said about the importance of the Olympic Gold.
Heres what Nadal said:
"Well, everything is different feeling,
but winning the Olympics is a little bit more special, because I know in tennis, the Grand Slams are more important than here, but here you only have one chance every four years."
Just before this years Olympics Federer Said :
"If I won, it would mean as much to me as a Wimbledon victory."
Federer: Olympic glory would mean as much as winning Wimbledon
If you search a bit you will find references of all the mens and womens gold medalists that say that they all rate it as their greatest achievement. All rate it on par or HIGHER than a slam. None rate it lower.
Just in this 1 article:
Gold Medals and Grand Slam Titles Fed, davenport, Agassi, venus all rate the Gold as highly or higher than a slam.
I like Agassi's quote in that article:
Agassi summed up his win by saying,
"To win a Grand Slam in the sport of tennis is the greatest accomplishment inside the sport. To win an Olympic gold medal is the greatest thing you can accomplish in any sport."