Does Unown G from Great Encounters pokepower stop Gallades Psycic Cut from being able to flip prize cards and add extra damage? Can you attach unown G to a benched pokemon?
No.
While flipping the prizes is an effect, it's not an effect done to the Pokemon that Unown G is attached to.
Other than that, the additional damage is still damage, so also not blocked.
Can you attach it to a benched Pokemon? Well, let's read it...
"...attach Unown G to 1 of your Pokemon..."
Nope, no restriction on where that Pokemon has to be located, so you can attach it to the bench.