There are a few reasons why store bought gnocchi might taste sour. One possibility is that the water used to make the gnocchi was too acidic. This can happen if the water has a high mineral content, or if it was left to sit out for too long and picked up impurities from the air. Another possibility is that the flour used to make the gnocchi was old and had gone bad. Finally, it's also possible that the gnocchi simply wasn't cooked properly and is therefore underdone. If any of these factors are present, it's likely that your store bought gnocchi will have a sour taste.