Brigette Supporter: Does Pokemon GX considered EX?
In Pokémon, new game mechanics are introduced from time to time, but this does not mean that they need to interact or be somehow backward-compatible. For most of the time, they are separate.

Pokémon-GX is a different card to Pokémon-EX cards and thus you can find up to 3 Basic Pokémon including Pokémon-GX cards with Brigette.