The location of my gym wouldn't be in a building, but rather a forest (Much like how Blaine's gym was inside a volcano/mountain)
My team would be grass types; Ferrothorn (Aurora), Cacturne (Harley), Vileplume (Erika), Cradily (Florina), Tangrowth (Operator) and Leafeon (Multipass)
The badge I would give is the Moss Badge, the TM I would give would be Solarbeam and as an extra gift, I would give the challenger herbal medicine/Heal Powder.