I think in this case 'fruit' without 'the' would be plural, but 'gerpe' is singular (accusative)
You're sentence would be with gerpī
It is not like that: "fruit" in the strict meaning is an abstract noun, not plural. If you say instead "Is the wolf eating the fruit?" for example, it is still not contemplated among the answers, but according to the course itself it should be. Gerpe = fruit / a fruit / the fruit / a piece of fruit / the piece of fruit. All singular, all correct.