When making gnudi that is tender to the tooth and not touch and chewy you need to use flour that is low in protein. Cake flour is ideal because it has somewhere from 6-8% protein but you may not have cake flour on hand. If you already are making your own pasta with 00 flour, that would be a great substitution as it has roughly 9-10% protein for a medium milled flour. If you have exhausted all flour options in your pantry, go ahead and use all-purpose flour. AP flour has about a 10-12% protein content and while it is almost double the protein of cake flour or 00 flour, AP flour gnudi is better than no gnudi, in my opinion.  Gluten-free flour can be substituted in in equal portions. My gluten-free friends recommended cup 4 cup brand of flour but use what you love.