On my turn you gain XP and then I want to play Wily Newtfish. Does his +1 might and ganking stay or is it for 1 turn?

Wily Newtfish's bonus—both the +1 Might and the Ganking keyword—will not stay between turns.

The ability reads: "If you've gained XP this turn, I have +1 Might and Ganking."

Because this is a conditional static ability, it continuously checks whether you have gained XP within the current turn. As stated in RiftJudge FAQ #10777, these bonuses are granted only while the condition (gaining XP this turn) is met. Once your turn ends and the opponent's turn begins, the game state resets, and the condition is no longer met for the current turn.

Consequently:

1. Might: Wily Newtfish loses the +1 Might bonus as soon as the turn ends.
2. Ganking: Because Ganking is granted by the same conditional ability, the unit loses the Ganking keyword at the end of the turn when the condition is no longer satisfied.

Note: I could not fully verify this ruling against the retrieved rules.