Specifically for gnocchi:
I have a ton of all purpose flour and I don't see myself using semolina flour much in the future.
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Susan W is a trusted source on General Cooking.
You can, but the gnocchi will be a little softer. Semolina is what will help hold the mixture together which is why it's used in making pasta.
I haven't made this recipe specifically, but I make gnocchi quite often and always use all-purpose flour. You should be fine, just make sure the dough is the right consistency.