I once thought , myself, that it may have come from Jack Webb's old flik, The DI, where the DI says something like, "Let me hear you roar, tigers!" --that was in 56/57, and the BS oohrah was tobegin a few years after that.
In my opinion, repeat: my opinion! Semper Fidelis, and Gung Ho! are the standby battle-cries of the Corps--and in the explanation that Evans Carlson gave to Robert Sherrod, aboard ship, on the eve of Tarawa, it seems to me that Carlson explained Gung Ho as an extension and/or explanation of Semper Fidelis--that, together w/General Puller's comments on Semper Fidelis, at the 1956 court-martial of S/Sgt Matt McKeon, tells the story!
Semper Fidelis, Marines!
Dick Gaines D