It also helps if you have a card made by Sony, if it's not made by Sony, it might or might not be useful. Personally, half of mine are used cards from EB or GameStop not made by Sony. they work fine, but I've had a couple of them start to go bad. To fix this, I'll put the card in slot 1 (sometimes slot 2 doesn't work for some reason), and then I'll hold it upwards with my finger. After this, the PS2 will be able to detect the card and I'll be able to save, still holding the card up.
Also, make sure your card is a 1x type (I forget the exact name). Gran Turismo doesn't like battery-powered multisave cards.