I agree with what Zeto said, and similar points made earlier. Strago's magic is clearly identified as Blue Magic, and if that is a basis to be built upon, then you can see Gau is not the same in his skill. Strago learns his when he observes the enemy's use of them, whereas Gau just runs off and has fun with the monsters and has to be found over and over again in the Veldt. Would get repetitive after a while.
Anyway, Strago's is far more Blue, but one can see why Gau's can be mistaken for it: since he emulates monster attacks (which aren't all magical, which is why the term 'Blue Magic' shoud be more specific)