Gardenia is the Grass-type leader of the second gym in Pokémon Brilliant Diamond and Shining Pearl. She doesn't have too many tricks up her sleeve, but the couple she does have can cause you some difficulties if you're not properly prepared.
Note: This is for the gym leader fight, not the rematch fight.
You'll have access to a solid variety of Pokémon by this point in the game. Add that onto the wide variety of types that Grass is weak against (Fire, Flying, Bug, Poison, and Ice), and you'll likely have at least one Pokémon able to handle Gardenia's organic team.
If you're still lacking one though, there are a few nearby options. Murkrow (for Brilliant Diamon players), Beautifly (by evolving Silcoon), Ponyta, and Zubat can all be found close by. So can Dustox (by evolving Cascoon), which is a very decent counter-pick to Gardenia's MVP Pokémon.
You can also head into the Grand Underground by this point, opening up a world of options. We've encountered both Crougunk and Skorupi down there before fighting Gardenia, both of which are fantastic options for Gardenia.
Items are also highly important if you're having trouble. Potions, Antidotes, and all the other healing items are sold for a reason—don't be afraid to take advantage of them.
Gardenia uses a lvl 19 Cherubi, a lvl 19 Turtwig, and a lvl 22 Roserade. We found that she'll use at most two Super Potions. Cherubi and Turtwig aren't that big of threats being pure Grass-types, but Roserade is a whole other story.
Your success in this fight will boil down to whether or not you can KO the already fully evolved Roserade. This suave menace is Poison and Grass-type, which means that your Poison and Bug-type moves will only do normal damage.
If you picked Chimchar as your starter, then your likely-evolved Monferno can deal with Roserade no problem. But if not, you may have some problems.
Roserade knows the moves Grass Knot, Petal Blizzard, Poison Sting, and Stun Spore. You can stop Stun Spore by using a Grass or Electric-type Pokémon but the best option is to keep a Poison-type Pokémon out. Poison resists all of Roserade's moves, greatly crippling its ability to deal damage.
Between the two Super Potions Gardenia will use and the Sitrus Berry that Roserade holds, it might be hard to finally KO ROserade before your own Pokémon succumb to its attacks. Be persistent in attacking and make sure to heal up when you're low. Do this and you'll be sure to take Gardenia down!
If Croagunk caught your eye as a means to beat Gardenia, then go check out Where to find Croagunk in Pokémon Brilliant Diamond and Shining Pearl on Pro Game Guides!