Correct! The second time depends on the random chance where the Ditto (now transformed into your Gyarados) will use the Mimic’ed Transform again. So as you guessed, you just wait until it procs a second time and then move on to the next step.
Damn, that’s awesome! I also have a dedicated False Swipe user for cases like this! Best move for capturing anything, really!
Of the 6 plushes I bought at Pokémon Center Tokyo DX, 4 were Regis! Namely Regirock, Regice, Registeel and Regigigas. They really spoke to me even though I’m not even a plush collector. Great colors and concept!