What is 00 flour and when should I use?
00 is extra-fine flour, the most refined grade. Here's a useful page for more about the old flour grading system. Other than pasta I use it for thickening sauces, and I sometimes make pizza dough with it, although I can't tell the difference between using 00 and good quality bread flour. I'd use 00 for Gnocci certainly as the chefs do, but most home cooks just use plain flour and seem to get good results. 

Basically, if you have it or can get it use it, if not plain flour is fine.