Birds have gizzards which they use for grinding up food with the aid of grit or gravel, some fish also have gizzards ie mullet, shad, also crockodiles and toothless anteaters but i dont know about opossums though ?
Opossums are omnivores and as such their digestive system is quite conventional, being very similar to a pig's with the obvious size difference... The critter is becoming more popular here in the U.S. as a household pet and adapt very well when raised from about 6 weeks of age... along with their natural affectionate nature they have no body smell and are easily trained to the litter box...