You can't 2HKO if there is a Lot of stuff that outspeed and OHKO you. Also, you may never be able to 2HKO Lugia, Ho-Oh, some Primal Kyogre Sets, and Possibly Cresselia without some sort of Special Attack Boosting move. :P
Also, why would you use Gyarados when Mega Salamence and Rayquaza are much better? Also, one (or Both) of them are easily available in Alola. :P