[rnhmjoj]

You're probably thinking of exotic matter with negative mass, not antimatter. Antimatter is matter made of (elementary) antiparticles, which are identical to the usual particles expect they have opposite electric charge.

So, we didn't expect there would be any difference as far as gravity is concerned.

[eigenket]

Antimatter has opposite charges for all charges, not just electric charge. Anti-electrons have opposite lepton number, anti-quarks have opposite flavour charges etc. This is true for all the charges we understand in the standard model.