Caputo Semora Limatinata 2.2 lbs (1 kg)

Q: is this the right flour for gnocchi a la romana?

No, I would not use semola. Instead use "00" flour, which you can usually buy at a local Italian deli/market. It has a lower protein content is is better for a past made with eggs, such a gnocchi. This flour is better for cavatelli, orecchiette and other pastas from southern Italy / Sicily that only use water. If you don't have a local supplier; paste this into Amazon.com search: "Antimo Caputo "00" Chef's Flour 2.2 Pound (Pack of 2)"

Q: Can you make sourdough bread with semolina flour?

You can. I've used this particular variety of flour in sourdough. There's a book called "Local Breads" that has a 100% semolina sourdough recipe, as well as a non-sourdough 100% semolina sandwich loaf. Whenever I want to make the semolina sourdough I'll split off some sourdough and feed it with semolina for a few days to cycle out all the regular flour and adjust the hydration to the specific recipe starter. Make sure that you're using refined semolina (rimacinata) as the coarse grain semolina usually used for pasta will behave very differently.