Electivire appears to be a recent addition to Gary's Pokémon arsenal. Not much is known about it but it has a powerful Iron Tail attack that was used in it's battle against Ash's Pikachu.

Obtained prior to episode: Home Is Where The Start Is

Umbreon is one of the Pokémon most thought of when thinking of Gary. After it evolved from Eevee, Gary & Umbreon shared a close bond of friendship, and this bond is shown when Gary uses it in battle. However Gary did not use it in his Silver Conference battle with Ash.

Evolved from Eevee prior to episode: Power Play

Having evolved from Wartortle, Blastoise has become the powerhouse of Gary's team. Managing to rack up a significant amount of wins in the Silver Conference, Blastoise is not a Pokémon to be messed with. Since Blastoise has been with Gary from the beginning of his journey, they share a close bond and Gary decided that Blastoise was the only Pokémon to take with him when he started his quest to become a Pokémon Researcher.

Evolved from Wartortle prior to episode: The Ties That Bind

Having evolved from Doduo at some point, Dodrio was only briefly seen within Gary's arsenal. In this appearance, Dodrio was used in order to stop Team Rocket from stealing an Aerodactyl. Not much else is known about it however.

Evolved from Doduo prior to episode: Putting the Air Back In Aerodactyl

Arcanine is another of Gary's Pokémon that we saw early on. Gary used Arcanine in his Gym Battle against Giovanni's Kingler in which Arcanine managed to overcome type weaknesses. However like Nidoking, it fell victim to Mewtwo. Gary has used Arcanine several times afterwards including in the Pokémon League, Silver Conference and an Extreme Pokémon Race.

Obtained Prior to episode: Battle for the Badge

Gary used a Tyranitar during one of the Trial Missions for Project Mew where he faced off against Articuno but it fainted quickly.

Obtained prior to episode: Trial Mission: A Frozen Raid Battle!!

Gary used a Hatterene during one of the Trial Missions for Project Mew where he faced off against other Trial members. It knows Psycho Cut.

Obtained prior to episode: Trial Mission: A Frozen Raid Battle!!

Gary caught Regidrago during his final trial mission for Project Mew.

Magmar is one of Gary's Pokémon that Gary used in his Silver Conference campaign. Magmar is a pretty powerful Pokémon with a strong set of attacks including Fire Blast. However, despite this, Magmar wasn't able to defeat all of it's enemies and fell victim to Ash's Heracross.

Caught prior to episode: The Ties That Bind

Scizor is another of Gary's Pokémon that wasn't seen up until the Silver Conference. Scizor is an outstanding bug Pokémon that Gary fully believes can overcome all foes. However despite this, even after beating many opponents, Scizor was made quick work of by a Charizard.

Caught prior to episode: Can't Beat the Heat!

Golem is another of Gary's Pokémon not seen until the Silver Conference. Being a powerful Rock Pokémon, Gary had hoped that Golem would be able to defeat Ash's Charizard, but despite Gary's best wished, Golem quickly fell to Charizard's powerful attacks.

Caught prior to episode: Can't Beat the Heat!

Nidoking was the first Pokémon of Gary's that we saw. Using it in his battle against Giovanni, Nidoking quickly wiped the floor of Giovanni's Pokémon. However it did surcome to the powers of Mewtwo. Gary later used Nidoking in the majority of his battles in the Pokémon League.

Obtained Prior to episode: Battle for the Badge

Krabby is one of Gary's Pokémon that we only saw briefly. When ringing Professor Oak about his own Krabby, Ash learned that Gary also recently caught one. However Gary's was several times larger than the Krabby that Ash caught.

Obtained Prior to episode: Mystery of the Lighthouse

Nidoqueen was first seen with Gary when he arrived in Pallet Town and defended the Lab from Team Rocket. After that, Gary used it in his Silver Conference Battles including his battle against Ash. It lost in the battle after a strong attack by Snorlax, however it did manage to put up a good fight.

Obtained Prior to episode: A Tents Situation

Nothing is known about Gary's Alakazam. We only know of it's existence through the screen on a computer showing Gary's Silver Conference Profile.

Caught prior to episode: The Ties That Bind

Nothing is known about Gary's Skarmory. We only know of it's existence through the screen on a computer showing Gary's Silver Conference Profile.

Obtained prior to episode: The Ties That Bind

Nothing is known about Gary's Kingdra. We only know of it's existence through the screen on a computer showing Gary's Silver Conference Profile.

Obtained prior to episode: The Ties That Bind

Nothing is known about Gary's Houndoom. We only know of it's existence through the screen on a computer showing Gary's Silver Conference Profile.

Obtained prior to episode: The Ties That Bind

Nothing is known about Gary's Fearow. We only know of it's existence through the screen on a computer showing Gary's Silver Conference Profile.

Obtained prior to episode: The Ties That Bind

Nothing is known about Gary's Pinsir. We only know of it's existence through the screen on a computer showing Gary's Silver Conference Profile.

Obtained prior to episode: The Ties That Bind

Where Gary obtained Eevee is unknown, but Gary & Eevee had an incredibly close bond. After Ash went through the Orange Island, Gary & Ash battled. Gary used Eevee against Ash's Pikachu, and due to their close bond and constant training, Eevee managed to defeat Pikachu. Due to the bond that Gary & Eevee had, one night Eevee managed to evolve into Umbreon.

Caught Prior to episode: The Rivalry Revival
Evolved into Umbreon prior to episode: Power Play

Doduo was seen briefly at Oak's Laboratory saying hello to Gary when he visited. We haven't seen Doduo in battle, however it would appear that it evolved into a powerful Dodrio.

Obtained Prior to episode: Showdown at the Poké-coral
Evolved into Dodrio prior to episode: Putting the Air Back In Aerodactyl

Squirtle was Gary's Starter Pokémon, given to him by Professor Oak. Squirtle wasn't actually physically seen in the anime, except for within a flashback to a battle by the Samurai. It evolved into a Wartortle and then later into Blastoise.

Given by Oak in episode: Pokémon, I Choose You
Evolved into Wartortle after episode: Pokémon, I Choose You