For the most part, the Gym Leader rosters of HeartGold and SoulSilver compared to Gold and Silver remain relatively unchanged, spare for a few tweaks. The teams are listed below.

Gym Leader #1: Falkner

Falkner is the first Johto Gym Leader and specializes in Flying-type Pokemon. He resides in Violet City, and for defeating him you receive the Zephyr Badge, a TM containing Roost, and the ability to use Flash outside of battle. Pidgey was originally level 7 while Pidgeotto was originally level 9.

Falkner, a Johto Gym Leader Pidgey Pidgeotto
Falkner Pidgey Pidgeotto
Types: Normal / Flying Normal / Flying
Levels: Lv. 9 Lv. 13
Attacks: Unknown Unknown

Gym Leader #2: Bugsy

Bugsy is the second Johto Gym Leader and uses Bug-type Pokemon at the Azalea City Gym. For defeating him, you receive the Hive Badge and a U-Turn TM. Each of his Pokemon are one level higher than the original games.

Gardenia, a Sinnoh Gym Leader Metapod Kakuna Scyther
Bugsy Metapod Kakuna Scyther
Types: Bug Bug Bug / Flying
Levels: Lv. 15 Lv. 15 Lv. 17

Gym Leader #3: Fantina

Fantina specializes in Ghost-type Pokemon and defeating her grants you the Relic Badge, Shadow Claw (TM65), and the ability to use Surf outside of battle. She resides in Hearthome City, and in Platinum, you can challenge her immediately after talking to her in the Contest Hall. Thus, she is now the third Gym Leader, and not the fifth like in Diamond and Pearl. Her team has changed as well, replacing Drifblim and Gengar with Duskull and Haunter (most likely because she is faced earlier in the game, and needs lower-leveled Pokemon). Ironically, although her outfit is designed after Drifblim, she has taken it off her team.

Hearthome Gym has been redesigned in Platinum. It is now pitch black, and wall of the Trainers within the Gym have flashlights. If one catches you in their light, you have to battle them. There are multiple doors inside the Gym, and each door has a symbol on it. Only one door leads to the next room, while the others teleport you back to the entrance. To determine which door is the correct one, you need to search each room for a second symbol and match it up with one of the doors.

Fantina, a Sinnoh Gym Leader Duskull Haunter Mismagius
Fantina Duskull Haunter Mismagius
Types: Ghost Ghost / Poison Ghost
Levels: Lv. 24 Lv. 24 Lv. 26

Gym Leader #4: Maylene

Maylene is the fourth Sinnoh Gym Leader and specializes in Fighting-type Pokemon. She resides in Veilstone City, and defeating her grants you the Cobble Badge, Drain Punch (TM60), and the ability to use Fly outside of battle. Her Pokemon choices have not changed in Platinum.

The Veilstone City Gym is different in Platinum, featuring a mess of exercise equipment. To reach Maylene, you need to swing punching bags into piles of junk to clear a path.

Maylene, a Sinnoh Gym Leader Meditite Machoke Lucario
Maylene Meditite Machoke Lucario
Types: Fighting / Psychic Fighting Fighting / Steel
Levels: Lv. 28 Lv. 29 Lv. 32

Gym Leader #5: Crasher Wake

Crasher Wake is the fifth Sinnoh Gym Leader and harnesses the power of Water-type Pokemon at his Gym in Pastoria City. By defeating him, you earn the Fen Badge, Brine (TM55), and the ability to use Defog outside of battle. His Gym layout and Pokemon choices have not changed in Platinum.

Crasher Wake, a Sinnoh Gym Leader Gyarados Quagsire Floatzel
Crasher Wake Gyarados Quagsire Floatzel
Types: Water Water / Ground Water
Levels: Lv. 33 Lv. 34 Lv. 37

Gym Leader #6: Byron

Byron is the sixth Sinnoh Gym Leader and specializes in Steel-type Pokemon. His Gym is found in Canalave City, and by defeating him, you earn the Mine Badge, Flash Cannon (TM91), and the ability to use Strength outside of battle. Byron is the father of Roark, the first Sinnoh Gym Leader who uses Rock-type Pokemon. The Canalave City Gym layout has not changed in Platinum, but Byron has replaced his Bronzor with Magneton.

Byron, a Sinnoh Gym Leader Magneton Steelix Bastiodon
Byron Magneton Steelix Bastiodon
Types: Electric / Steel Steel / Ground Rock / Steel
Levels: Lv. 37 Lv. 38 Lv. 41

Gym Leader #7: Candice

Candice is the seventh Sinnoh Gym Leader and utilizes Ice-type Pokemon. She resides in Snowpoint City and awards players the Icicle Badge, Avalanche (TM72), and the ability to use Rock Climb outside of battle. Like other Platinum Gyms, her’s remains identical to Diamond and Pearl‘s. However, half of her team has changed; she has replaced Snover and Medicham for Pilowswine and Froslass. She also uses one more Pokemon on her team than the six Gym Leaders before her.

Candice, a Sinnoh Gym Leader Sneasel Piloswine Abomasnow Froslass
Candice Sneasel Pilowsine Abomasnow Froslass
Types: Dark / Ice Ice / Ground Grass / Ice Ice / Ghost
Levels: Lv. 40 Lv. 40 Lv. 42 Lv. 44

Gym Leader #8: Volkner

Volkner is the final Sinnoh Gym Leader and harnesses Electric-type Pokemon at the Sunyshore City Gym. By defeating him, you earn the Beacon Badge, Charge Beam (TM57), and the ability to use Waterfall outside of battle. His Gym has not changed since Diamond and Pearl, but he has replaced two members of his previous team for more Electric-types: Ambipom and Octillery for Jolteon and Electivire. He is the only Sinnoh Gym Leader to have a full mono-type team.

Volkner, a Sinnoh Gym Leader Jolteon Raichu Electivire Luxray
Volkner Jolteon Raichu Electivire Luxray
Type: Electric Electric Electric Electric
Levels: Lv. 46 Lv. 46 Lv. 50 Lv. 48