I think many previous posts said that those cards are very good. This is the case if you look at Flareon, but the others may not be that great. Let me look at the cards:
Eevee GX
+ Can evolve into any Eeveelutions
- Can't evolve right away like Energy Evolution Eevee
- Weak to fighting types
- Gives away 2 Prize cards
- easy to KO
- No really useful attack, they are pretty standard
3/10. You rather play Energy Evolution Eevee.
Jolteon GX
+ HP are okay
+ No retreat cost
+ Nice GX attack, could do some more damage though
+ Head Volt can 2HKO but needs two attachments. If you pair it with some accleration like Tapu Koko Prism Star or the Lightning Prism Stadium it could be good
+/- Elecbullet is a nice 1-energy attack but it's also on Buzzwole which is a Basic that hits relevant weakness. Could be stronger.
- Weak to fighting types
Overall I'd give it a 6,5/10. It is decent but not meta defining.
Vaporeon GX
+ 210 HP
+ No that relevant weakness
+ Ability can prevent 2HKOs if you use more than one each turn (You could even heal 120 with 4 Vaporeon in play [or 170/220 with some Leafeon GX that evolved from Eevee GX and are retreated/switched afterwards])
- Hydro Pump: great if you can pair it with some accleration. Hits 40+90 = 130 if you use [W] energy to fulfill the attack cost.
+ Healing Shower is great against any kind of spread damage decks
- 2 retreat cost
- there is no good accleration to pair it with in standard
I'd say 6/10 because it is missing the energy accleration. If there was some good in standard I'd give it 8/10.
Flareon GX
+ 210 HP
+ Heat Stage is a really great attack that is a lot better than Reshiram's GX attack because it can be powered up and used in one turn and you can use it multiple times. Also, you use the 30 damage to prepare OHKOs.
+ Bright Flame is a good attack OHKOing a lot of cards with Choice Band and/or 30 damage from Heat Stage. Still, we need to know how many energies move to the discard pile.
+ No popular weakness
- Power Burn GX seems not so strong (it may have an additional effect we don't know)
- You'll need to Heat Stage once before Bright Flame, making it basically a 2HKO
In my opinion, Flareon is the best of the cards and can find it's place in the meta. 8/10!