Make sure you haven't pre-cooked your gnocchi. This recipe allows for plenty of time for the gnocchi to completely cook in the oven right in the sauce. If you pre-cook it, it will get overdone and mushy. If that isn't the case, try baking for a little less time and see how that leaves the gnocchi's texture. You could always try swapping the brand of gnocchi that you're using as well.