Does Unown G's "GUARD" Poke-POWER prevent effects from attacks that the Unown G is attached to?

No, the Poke-POWER only prevent effects from an opponent's attacks, not your own. Unown-G's "GUARD" Poke-POWER should say, "As long as Unown G is attached to a Pokemon, prevent all effects of attacks, excluding damage, done to that Pokemon by your opponent's Pokemon".

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