From Bulbapedia, the community-driven Pokémon encyclopedia.
An example of a Pokémon-GX
Pokémon-GX (Japanese: ポケモンGX Pokémon GX), officially written Pokémon-GX, are a variant of Pokémon in the Pokémon Trading Card Game. They were first introduced in the Sun & Moon expansion (the Collection Sun and Collection Moon expansions in Japan). Pokémon-GX have a stylized graphic on the card name.
Pokémon-GX work very similarly to both Pokémon-EX from the Black & White and XY series and Pokémon-ex from the EX Series. They possess considerably higher HP and stronger attacks compared to their regular counterparts. However, when a Pokémon-GX is Knocked Out, the opponent takes two Prize cards instead of one. Also like Pokémon-ex, Pokémon-GX are classified with their usual stage of Evolution, contrary to Pokémon-EX which had fully-evolved Pokémon as Basic Pokémon.
The main trait that makes Pokémon-GX differ from previous variant Pokémon is their exclusive mechanic called GX attacks. These attacks are based on the Z-Moves mechanic introduced in Pokémon Sun and Moon and like them, have more powerful effects and/or damage output than normal attacks. However, a player can only use one GX attack per game. A special GX marker is used to take note of whether the player has already used its GX attack or not.
Aesthetically, the artwork on Pokémon-GX covers most of the card, with the background illustration fading into the top and bottom-most areas, while still having the Pokémon break the card's borders (following on previous Pokémon variant cards). As such, the card text presented across the artwork is outlined, resembling Full Art cards. A bar containing the GX attack name separates it from the other attacks and the effect text is colored blue. Finally, the card border features a black line running over it.
Pokémon-GX released on main expansions and sub-sets are usually accompanied by Full Art and Secret Hyper Rare prints.
Pokémon that have appeared as Pokémon-GX