This is the instruction (fleshed out by me) for extending the Ditto Trick and using it forever:
1. Fly away from the Gambler as usual.
2. Enter the pokemon center, go and use the PC, change boxes to save the game.
3. Reset! The Start menu works again, and you can do everything normally, including fighting by talking. The game is normal and the Ditto trick is halfway through; its conclusion can be postponed as long as you like.
4. Fight the Elite Four.
5. Find a Ditto, let it Transform, run, fly to Saffron or Lavender.
6. Go back to the Gambler's route, press "B", and fight your chosen pokemon.
I tested the underlying glitch on the standard Gambler and a regular trainer on my non-glitchy physical Blue cartridge.
My Blue isn't ready for the Elite Four, so I tested on the first Scientist in Silph Co., on floor 2.
I had forgotten to reset, so the game crashed when I talked to him. :P
I reset, spawned by the PC, and decided to see the Gambler to test. He didn't see me. So I went up to him and talked to him. The battle went normally. Afterward, I walked around a bit.
The START menu did not work anymore.
In the area 2+ spaces above him, I was walking slowly (like a 1/2 second pause on each tile), and any time I stopped moving, he became invisible (!).
EDIT: It seems all trainers on the route are glitch invisible.
I walked over to the middle of the route, where the group of 4 trainers stand. Before I got to them, I was forced to stop. The route music changed to an infinite loop of scrambled Pokemon cries. I could only move 1 tile every 5 seconds (omg, so slow…) by holding the arrow down continuously.
EDIT: In another test, I ran to Saffron and flew to Lavender, reentering from there. When I got near the Group of 4, the screen went black, so I reset.
I went back above the Gambler. He was still glitchy invisible. Emo Gambler ftw.
I left the route to Saffron. Upon returning, the route music was the regular one punctuated by pokemon screams from the previous loop. O_O
I reset & tested this again several times. Each time was the same, except I landed in a different tile near the group of 4, and a different choice of cries played in the loop.
EDIT: New testing shows that any audio can be stuffed into this loop. I heard the Lavender Town music.
In hindsight, I seem to have caught the same glitch as the Cave of Dragonflies author: "the whole route seemed to glitch up". I suppose having to beat the Gambler twice is a difference of versions.
I am afraid to test further, as my Blue is my only version with the Gambler unbeaten, and I don't want to have to restart. :P
I got out of this by resetting and fighting the Scientist normally (letting him see me), then finding a Ditto (for a Hitmonchan). However, fighting the Scientist by talking will not pop up the Start menu upon return to the route. Fighting a second trainer by talking also does nothing at the route.
EDIT: New testing shows that if you fight a trainer by talking and then a trainer by sight, the Ditto trick can be executed normally. So, it is quite likely the E4 will do.
I suppose your Rival would count as not being adjacent to you, because you're pulled to him.
0 ERROR variation steps:
1. Fly away from the Gambler.
2. Save by changing boxes on a PC.
3. Reset. START menu works again.
4. Go fight the Gambler. START menu disabled again…
5. Go to the Group of 4 in the middle of the route.