How come next gen pokemons are available on gamepress?

Because there is a formula to calculate the pokemon go status, and the formula is based on the original pokemon games, so in fact you can know the pokemon go status of any existing pokemon, even the x y ones, just look their status in the original games and throw them in the formula

Pokemon GO stats are based on the stats from the main game, and the conversion formula is solved. Because of this, we can predict what stats next gen Pokemon will have reliably. You can read more about them here:

https://pokemongo.gamepress.gg/pokemon-stats-advanced