Galvalume is a mild steel product that is dipped in aluminum. There is really nothing too difficult about cutting it. Treat galvalume that same as you would stainless steel and cut with nitrogen. Many fabricators have seen little success trying to cut galvalume with oxygen. Just be sure to factor in some extra cutting time and run the job 15%-20% slower than normal.
There are times with when the thickness of the aluminum coating can vary quite a lot which will inevitably affect your pierce condition, cutting condition, feed rate and finished product. It has also been my experience that the product can vary greatly from one supplier to another.