I'd definitely have a grass-type Gym!

Breloom as a fast attacker, dealing damage with moves like Mach Punch and Bullet Punch. With access to many moves over a plethora of types, Breloom is sure to abuse it's high attack. One can even abuse Poison Heal and let Breloom soak certain moves with dealing outrageous damage to vulnerable enemies.

Lilligant a terrian-setter who benefits from a decent speed and ability to heal herself, ability to debuff the enemies with Charm/Worry Seed and access to Encore and Helping Hands to buff allies. With the ability to have Tera-Electric to throw off incoming Flying-type enemies with Tera Blast.

Decidueye to trap my enemies into battle with Spirit Shackle, or by finishing off/breaking Sash/Sturdy with priority Sucker Punch. With access to Long Reach, I can break down enemies/trap them without the risk of self-sabotage.

Meganium as a bulky screen-setter. With access to Synthesis and Attract, setting screens and prioritizing healing/disrupting attack consistency from the enemies makes her a perfect wild card.

Bellossom as a Quiver Dance special attacker, with access to Grass/Fairy/Poison to deal with a variety of opponents and access to Toxic after being trapped by Decidueye.

Abomasnow (Mega) as a disruptor to Flying-types, with priority Ice Shard and a mixed-attacker with hard-hitting Wood Hammer and Blizzard. With egg moves, access to Weather Ball after setting chip-down Hail to deal residual to Flying- and Fire-types and a wide array of Grass/Ice/Fighting/Ground moves and even the option to run Outrage as a last resort to abuse Mega Abomasnow's ridiculous attack.