I measured the single and double Gaggia Classic baskets here as (IIRC) 58.4mm, ie they should just accept a 58mm tamper.
I've bought a Motta taper for 58mm baskets which measures as 57.5mm. The very slight extra room allows a some grounds to rise at the sides which I see as beneficial in some respects, especially if the grounds haven't been levelled perfectly.
It works perfectly well, and importantly to me, is high quality and made in Italy. All of the Motta stuff is Italian-made from what I can see.
I think people may think into stuff like tamping a little too much. Look at me, I bloody measured the baskets and tamper with digital calipers!