The only part of the grape that is toxic is the "seed".
All my dogs have gotten grapes(seedless)as treats.I'm not talking every day and a whole bunch.And I make sure they are washed first.
My first GSD lived to be 13.With absalutely no health issues leading to the grapes.And he started getting them as treats when he was 3 months old.My current GSD is almost 10.He too has been getting them since he was 3 months old.He is in great health except for a bit of arthritis.
Also,I really don't think my breeder/vet. would have given me the ok for them if she/he knew it would cause major health issues.