Finally together, here are all 8 gym leaders from Gaiien! You can read more about them on their individual portrait posts.
Tier 1: Luciana “Tierra” Rodriguez – Ground – Dust Badge – Umber Village
Tier 2: Alex “Hawthorn” Wong – Grass – Seed Badge – Verdure Town
Tier 3: Bryony “Belladonna” Jourdain – Poison – Venom Badge – Porphyry City
Tier 4: Brent “Pyre” Carter – Fire – Pyre Badge – Russet Town
Tier 5: Galen “Lord Ironhelm” Richter – Steel – Gear Badge – Port Brac
Tier 6: Mikoto “Tsukuyomi” Arata – Ghost – Oblivion Badge – Sere Island
Tier 7: Genevieve “Nocturna” Park – Dark – Starlight Badge – Sunset Mountain
Tier 8: Matti “Polaris” Virtanen – Ice – Frost Badge – Sastruga Fjord
AKA "Ordinary Ground-type Leader", "Asshole Grass Leader", "Super Asshole Poison Leader", "Used-to-be Spooky Fire Leader", "Dismissive Robot Guy", "Ser Not Appearing in This Fanfic", "Life Ruined By Demons", and "Sans Undertale"
The "tier" system refers to the approximate level/skill/pokemon diversity range expected in the match with the gym leader, with level ranges of 25-30, 30-35, 35-40, 40-45, 45-50, 50-55, 55-60, and 60-65, respectively, and 65+ for S-tier (or trainers with 8 badges). (Subtract 10 levels for most other regions that don't have an age 18+ league).
In the most populous regions like Kanto and Unova, the gyms see so many challengers that the leaders often have seconds to take matches, and have a wide variety of pokemon of different levels to support challenges at any tier. Gaiien is less well-traveled and requires challengers to stick to either the designated Tier or S-tier (every gym leader will have a personal team that can handle and is frankly eager for a battle at S-tier).
That said, most of the gym leaders have some higher-level pokemon kicking around too strong for their own tier and not quite ready for an S-tier match due to level decay, so many gyms can still accommodate a mid-tier challenge if needed, but it's recommended that you set up the match by appointment with the leader.
"Levels" are real in the Gods and Demons universe but they're the result of aura studies of hundreds of pokemon; your Ivysaur reaching level 32 doesn't guarantee it evolving to Venusaur, but on the spread of "level" of evolution of every Ivysaur ever scanned by a Pokedex, the average timing of evolution was about Level 32. Similarly, you won't get in trouble for using pokemon that over- or underlevel a tier in a match, although you might be thrown out by the referee if you pull out a very underleveled pokemon. The level range is for the gym leader, not the challenger.
Pokemon level caps out at about level 70 in the Gods and Demons universe, with 100 as a theoretical maximum achievable by constant absorption of type-compatible energy. Legendaries with few on-record pokedex scans or kaiju pokemon can trigger a pokedex to report a level above 100, or even one that overloads the counter and appears as gibberish text.
Seeing them all together, I sort of wish that I’d put them in more strongly-themed or brightly-colored outfits to make their type association more obvious, but this is how I intended the leaders to look for the fanfic, i.e. more like more ordinary people with differing amounts of commitment to their gym leader Brand (and hence the stage names instead of just strangely coincidental first names by type affinity).