As Johnny Phantom said, you can only attach Unown G as a Tool card, not as an energy. While attached it prevents all effects of an attack done to it. What this means is, anything thats not dealing damage on the opponets attack it usually prevented. I say usually because there are cards like... off the top of my head, Dragonite ex d from DF. Its first attack says that the opponet cannto play trainer cards from his/her hand during their turn. Unown G does NOT prevent effects such as this, because the effect is on the player, not the Pokemon.
Hope it helps.