Seems according to Serebii, the Gym Leaders reappear in the Survival Area. It doesn't sound as though they all gather there at once.
Roark:
Aerodactyl Lv. 62, Probopass Lv. 61, Rampardos Lv. 63, Golem Lv. 61, Tyranitar Lv. 65
Gardenia:
Jumpluff Lv. 61, Cherrim Lv. 62, Bellossom Lv. 61, Torterra Lv. 63, Roserade Lv. 65
Maylene:
Hitmontop Lv. 62, Breloom Lv. 62, Medicham Lv. 63, Lucario Lv. 66, Machamp Lv. 64
Fantina:
Banette Lv. 61, Mismagius Lv. 63, Gengar Lv. 65, Drifblim Lv. 61, Dusknoir Lv. 64
Byron:
Skarmory Lv. 61, Magnezone Lv. 62, Steelix Lv. 61, Bastidon Lv. 63, Aggron Lv. 65
Candice:
Weavile Lv. 62, Abomasnow Lv. 61, Mamoswine Lv. 61, Froslass Lv. 63, Glaceon Lv. 65
Volkner:
Jolteon Lv. 61, Raichu Lv. 61, Luxray Lv. 62, Electivire Lv. 65, Lanturn Lv. 63
Hasn't seen Wake yet, thinks something has to trigger it. Apparently Stark Mountain now much more resembles a volcano, and you have some run ins with Team Galactic.
Known (new) Superpower learners:
Flareon, Golem, Donphan, Honchkrow, Armaldo, Ursaring, Luxray, Hippowdon, Mamoswine, Snorlax, Tyranitar, Blaziken, Absol, Rampardos, Dragonite
Known (new) Outrage learners:
Venusaur, Marowak, Rhyperior, Tauros, Lapras, Gyarados, Snorlax, Kingdra, Nidoking, Flygon, Salamence
Known (new) Aqua Tail learners:
Gliscor, Seviper, Quagsire, Drapion, Walrein, Lapras, Salamence, Swampert, Slowbro, Slowking, Armaldo, Rhyperior, Golduck, Tyranitar, Garchomp, Floatzel
Known (new) Heat Wave learners:
Crobat, Togekiss, Zapdos
Known (new) Seed Bomb learners:
Jumpluff, Carnivine, Shiftry, Victreebel, Vileplume, Ambipom, Snorlax, Cradily, Donphan
Known (new) Gunk Shot learners:
Infernape, Donphan, Ambipom, Snorlax, Slaking, Toxicroak
A gross number of things are already known to get Signal Beam.
Only known new Vac-Wave at the moment is Poliwrath. Meh. I kinda want Porygon-Z to get it.
Exeggutor gets Zen Headbutt, about fucking time. Ditto Swampy Aqua Tail. I'm not saying they needed them, it just made perfect sense.
There is definitely an Earth Power move tutor. Torkoal, Swampert, Garchomp, Whiscash, and a mess of other ground types may now learn it.
I've been looking over the enormous document detailing Battle Frontier pokes, and after some time and research I've been noticing some trends, that lead me to the conclusion that while this doesn't mean there aren't more recipients of these moves, these are in fact ALL of the possible encounters. Most third-stage evolutions have four possible movesets, one exception being Azumarill with only one; lesser stuff like Tropius has two movesets; the first stage forms have only one, and the incredibly worthless things like Magikarp and Caterpie have none. Of the novelties, Smeargle and Ditto are absent; Castform and Chatot have one set.
Altogether this totals to 950 encounters.