Dragon is the one GLC type that can't just run 13-16 copies of a single Basic Energy and call it done. Dragon-type attacks in the Pokémon TCG have always demanded mixed energy types in their costs, which is a real deckbuilding problem in a singleton format — Twin Energy, the most obvious fix, is banned in GLC. That leaves Double Dragon Energy as the type's one specialized answer, and both established Dragon builds are structured around making that single card count.
The upside of that difficulty is raw damage: Dragon attackers consistently hit harder than the mono-energy alternative a mixed cost would suggest, which is why the type has a reputation as GLC's glass cannon — enormous swings if the energy lines up, dead cards if it doesn't.
Double Dragon Energy provides two Energy of whichever types a Dragon attack needs, which is what makes running Fighting-and-Water or Water-and-Darkness costs in the same 60-card deck workable at all. Because it's singleton like everything else, a Dragon deck's Basic Energy split has to be built carefully around what its attackers actually cost turn to turn, not just split evenly.
Garchomp Blades runs a Fighting-and-Water Energy base: Garchomp attacks cheaply off that split, Haxorus cleans up bigger threats, and Noivern disrupts opponents leaning on special energy of their own. It's the more aggressive of the two builds, trading blows early rather than setting up.
Goodra Wall takes the opposite approach on a Water-and-Darkness base: Goodra's high HP lets it tank hits while the rest of the deck develops, Hydreigon converts Darkness Energy into spread damage across the board, and Alolan Exeggutor can attack with whatever Energy happens to be attached — a safety valve for the awkward hands a mixed-energy deck is prone to.
The single point of failure is obvious but worth stating plainly: prizing your one Double Dragon Energy, or drawing the wrong half of your Energy split, can strand an attacker that's otherwise ready to swing. Both established builds lean on their bulkier or cheaper Pokémon (Druddigon, Turtonator, Drampa) as a bridge for exactly that scenario, rather than assuming the energy will always line up on schedule.
Full decklists for both builds are on the GLC hub. If Dragon's energy requirements feel like a lot to take on first, Fire and Water are more forgiving starting points, and what Gym Leader Challenge actually is covers the format's rules from scratch.