If cost is an issue, you could have a look at some of the budget metal decks people around here have come up with. If what you are looking for is a real hard counter to Gardevoir-GX because it's popular at your local league, then metal really is the way to go. Below is an example of a somewhat 'budget' deck that would be excellent fun to play against Gardevoir-GX, is easy to wrap your head around AND brings the cool factor in Solgaleo. I play a variant of this deck at my local club and the kids there really respond to it.
Cost: (Rounded) $50.00
##Pokémon - 15
* 1 Oranguru SUM 113
* 4 Rayquaza GRI 106
* 3 Solgaleo-GX SUM 89
* 1 Magearna-EX STS 110
* 4 Cosmog SUM 64
* 2 Cosmoem SUM 65
##Trainer Cards - 32
* 4 Professor Sycamore STS 114
* 4 Ultra Ball PLF 122
* 2 Lysandre AOR 78
* 2 Skyla BCR 134
* 4 Rare Candy SUM 129
* 2 Super Rod NVI 95
* 2 Nest Ball SUM 158
* 2 Field Blower GRI 125
* 3 N NVI 101
* 2 Choice Band GRI 121
* 3 Max Potion BKP 103
##Energy - 13
* 9 Metal Energy HS 122
* 4 Double Colorless Energy EVO 90
Total Cards - 60
The goal of this deck is too use Rayquaza's Turbo Storm to power up Solgaleo then use Solgaleo's ability to switch Solgaleo into the active, rinse and repeat.
Now, this list isn't entirely up to date anymore. Lysandre has been replaced by Guzma in the new standard meta and Magearna-EX isn't really played at all anymore. I also think that 4 Rayquaza is a bit much and you would probably be better off playing 2 of those and 2 Registeel from the Crimson Invasion expansion, which will one-hit KO a Ralts and accelerate an energy to boot. Solgaleo-GX is great but if it's too expensive or hard to find then its non-GX variant from Guardians Rising also comes highly recommended, although I would suiggest playing
at least 1 Solgaleo-GX for its fantastic ability which really drives the deck.
Even if you guys are just starting out with your collection it should be fairly easy to trade for these, or cheap enough to buy what you don't already have. Solgaleo-GX is, in my opinion, the best card that NOBODY is playing and it's fantastic against Gardevoir. It's about to get a whole lot better in the next expansion, as well. A lot of new cards coming out in february will fit right in to this deck, so you are definitely getting some mileage out of it.
EDIT:
Here's the exact list I use at my local cup. It's got Tapu Lele-GX in it as well as Drampa-GX, but that card is the least used in my deck. Replace that with Registeel (or two, though you'd need to drop another card for it) and your boy will have a strong opening against Gardevoir.
Pokémon: 16
- 4 Cosmog SUM
- 3 Cosmoem SUM
- 3 Solgaleo-GX SUM
- 1 Solgaleo GRI
- 2 Tapu Lele GRI
- 1 Drampa-GX GRI
- 2 Rayquaza GRI
Trainers/Supporters: 31
- 3 Rare Candy SUM
- 4 Ultra Ball SUM
- 2 Heavy Ball BKT
- 1 Nest Ball SUM
- 2 Choice Band GRI
- 3 Field Blower GRI
- 1 Super Rod BKT
- 1 Max Potion GRI
- 4 Professor Sycamore BKP
- 3 N FAC
- 1 Hala GRI
- 2 Skyla BKT
- 3 Guzma BUS
- 1 Brigette BKT
Energy: 13
- 9 Metal Energy
- 4 Double Colorless Energy
An added bonus of this deck is that Solgaleo-GX can one-hit KO every Pokémon in the entire game, bar none. There's only a couple that have more than 230HP and even then all it takes is a Choice Band for that bit of extra damage.