I think Leafeon-GX is being overhyped. It's a great card in theory, and there's a lot of Pokémon you can pair it with to take advantage of it's GX attack, but you need the perfect start every turn for it to be worth it. If you whiff on the Turn 1 Brigette and struggle to put Pokémon on your bench, it's GX attack is lackluster. You also need to start with Eevee, or have a way to get it into your active if you don't start with it, which won't happen every game. Once you've used it's GX attack it's just meh the rest of the game because you'd rather focus on the Pokémon you paired it with. You gotta run a 4-3 line to maximize your chances of starting with it which is a lot of space for a Pokemon your mainly only going to want to use Turn 1. I don't know, I could be wrong and it could run great but the people at my League that have tried it have all said that it's clunky and inconsistent.
Clearly the title is not ''does Leafeon have the tier 0 spot in the meta'', so I really don't get all the people who argument the post saying it's not a monster of a card.
The ability is rarely if even used, the attack is cluncky and really costly, the GX is good to set up, as you yourself stated, and that's the point.
Pairing with golisopod, You can Easily run 2 Leafeon.
Now, talking numbers, playing a 4-2 split leaves you with about 39% chance of getting it as your starting Pokémon.
Using a 4-3 of Golisopod you have 8 Pokémons as ideal starters, since Wimpod has free retreat basically, 70% chances of getting one as the starter (with 2 float stones, we get to about 80% of having an active Pokémon and being able to retreat).
With 4 Ultraball, 3 Brigette, 2 Minor Ball (used to maximize the effect of the attack), 2 Lele to get the brigette, we have 75ish % chance to get one in the opening hand, removing the dead hands with no basic it's about 70% I think (Don't remember, made these calculations ages ago and trust me when I say I don't have the patience to run them again.).
Basically 3 out of 4 games we have our Eevee in the active spot, most of which by using a Brigette.
Yes, it's a setup deck, Parallel City is a pain in the ass as is Cyrus, yes, we fill the deck with cards to optimize the combo and thus feel the lack of some items.
And focusing on the Pokémon you pair with Leafeon is kind of the point, since after the GX the only use it has is the occasional Guzma-> Pick it up and heal-> Retreat with float stone to hit 190 with Golisopod.
Tried it at my league, it's fairly consistent but the problem is that, even tho I built it with Lurantis and made it run most of the time, Golisopod stole the spotlight and it suffered without an Ideal setup more than let's say a Zoroark deck, reason why I cut Lurantis and added Golisopod, since it runs by itself, hitting 170 with a lone Lurantis promo in play and picking off Leles.
Don't underhype cards just because people overhype them.