Finely dice the pumpkin flesh and place in a saucepan with the preserving sugar.
Squeeze the lemons and add the juice. Bring to a boil while stirring and then cook over low heat for at least 7 minutes.
Fold in the grappa, fill the jam on the spot into twist-off jars and seal. For pumpkin jam of any kind, a little longer cooking time is recommended, otherwise the jam can easily become too thin.