Gary's journey through Kanto was much different than Ash's as you may know. Gary acquired 10 Kanto Gym Badges 3 of which were from the same gym as Ash. Gary's first Pokemon was Squirtle who Prof.Oak gave Gary before he left Pallet Town on his journey. Gary on his way to Pewter City was challenged to a battle against Samurai a bug pokemon trainer who had a Metapod(That only knew harden) and a Pinsir. Gary Defeated Him And Went on to challenge the Pewter City Gym Leader Brock and his pokemon team of Rock type pokemon, Geodude and Onix and won very easily with Squirtle.
Gary then left from Pewter and most likely took a different route to Cerulean City around Mt.Moon and through a city where he fought another Gym Leader most likely a Fighting Type Gym was in the town and the Gym Leader had a Primape, Machop and or a Poliwrath and would have been a Black Belt or even Bruno.
Now when Gary arrived in Cerulean City the Gym Leaders at this time would have been 3 of the 4 Sensational Sisters(Misty is Absent at this time). 2 of the sisters(The Lesser Lily and Violet) would be normal trainers in the gym and Daisy(The superior) would be the Gym Leader. Daisy's Pokemon Are Dewgong or Seel(If Gary arrived before ash they would have a Seel, After Ash would be a Dewgong, Gary Most Likely arrived After Because He went Arround Mt.Moon as Proposed as the above Theory) A Seaking and possiably a Shelder or Goldeen.
Gary most likely skipped the Thunder Badge(Shown in "The Battle of the Badge") and went directly to Lavender Town and Gone through the rock tunnel and such. Gary would have Arrived Before Ash in Lavender Town and there may have been a gym in the Pokemon Tower run by Agatha[/COLOR]. Agatha's Pokemon team would have been A Gengar a Goldbat and a Haunter or Arboc.
Gary would have continued his journey through Kanto and gone to Celadon City where he would have challenged the Gym Leader [COLOR="rgb(46, 139, 87)"]Erika[/COLOR] and her pokemon team of [COLOR="rgb(46, 139, 87)"]Tangela[/COLOR], [COLOR="rgb(46, 139, 87)"]Weepinbell[/COLOR] and [COLOR="rgb(46, 139, 87)"]Gloom[/COLOR]. Gary would not have been in the pressence of Team Rocket because team rocket invades after the Celadon Gym Burns down and Ash wins his Badge, but it is possiable that Gary runs into Eusine the suicune hunter.
Now keep in mind that Gary never obtained the Soul Badge or the Marsh Badge. It is a theory that Gary went back to Viridian Forest where he rechalenged Samurai who was now a Gym Leader of Bug Pokemon. Samurai at this time would have had a Pinsir still, a Scyther and a Butterfree. Evidence also shows that it is possiable that Gary obtained a Scyther in the Viridian forest and or a Pinsir. This part of Viridian Forest would not be the same side he went through going from [COLOR="rgb(0, 100, 0)"]Viridian City[/COLOR] to Pewter City. This side would have been one the far west side by the sleeping Snorlax. From here continuing on he most likely went to the Seafoam Islands where he challenged a [COLOR="rgb(0, 255, 255)"]Ice[/COLOR] Type Trainer Most Likely Lorelei(Prima Stupid 4Kids lolz) and her team would have consisted of Slowbro, Jynx, CLoyster and Dewgong. After this Lorelei would have left Seafoam Islands to goto either Sevii islands or to the Indigo Plateau.
This is all i have so far except that there are 3 more gyms in Kanto. there is a Dragon Gym, a Flying Gym and a Normal Type Gym.