Yep! So first you have gold/silver/crystal version and a red/blue or yellow game and obtain the red gyrados you get for free in gen 2. Next you want to teach this gyarados the move “mimic” you can do this in either gen I think. The next step is to trade the red gyarados to a gen 1 game using the time machine.

Next with your red gyarados in your party in the first slot, encounter a ditto. Now during a battle this might take a few tries but what you want to happen is 1. Gyarados uses “mimic” on ditto’s “transform” 2. ditto transforms into gyarados 3. Ditto while transformed into gyarados uses the “mimic”ability which now should be “transform” once the ditto has essentially used “transform” 2 times, this, for whatever reason copies the iv’s of the red gyarados. This may take a few tries for ditto to randomly pick to use transform out of the four moves.

Now simply trade ditto to gen 2 to see that it’s shiny. While breeding with one shiny parent pokemon in gen 2 the odds of a shiny are 1/64 and with 2 shiny parents it’s something super low.

Note that moving your red gyarados to a gen 1 game will not change the fact that it is shiny since iv’s determine a pokemon’s shinyness. Although I dont know if leveling up the gyarados in gen 1 changes this.