Gamespot is just biased towards Sony. They love Sony and everything Sony related, Gamespot wouldn't do anything to hurt Sony in the least unless someone there was having a nervous breakdown and were out of control or something. Not a joke here.
But GS can give Nintendo games very good scores sometimes. Most of the time though they let Sony and Sony partners get away with bad things and overrate other things and sometimes they're very hard on Nintendo. That's pure bias. However when a game is not seen as direct competition to anything Sony they allow themselves to give the game a good score, even when it's shit and that's not just true for Nintendo, it's(was) Sega, Microsoft, Microsoft's partners(tecmo) and so on. Sometimes they have to give hot titles a good score, but they always make sure it's a notch down and the opposite is true to hot Sony titles, it's a notch up. The bias is toward Sony. For example their second(or third) perfect score was Chrono Cross and they gave 9.9 to Soul Calibur, now I'm not sure it's still the case because sometimes they regret their judement and change it after the damage is already done to get away with it but anyway. Compared to Soul Calibur for its genre Chrono Cross definitely didn't deserve that perfect score and that's just one of many examples.
I've been a reader of Gamespot ever since the begining of their existence and it's always been like this. To whoever said Nintendo games might actually suck, that may be true to their Gamecube titles, yes, but if you mean that in the DS case then you obviously don't know Gamespot.