I have not found a trap with a G Power number either, but I believe they make them.
When a Trap is placed on a Gate card it will open revealing either a different Attribute or G-Power. In a notmal battle, you woulld play with 3 Bakugan plus 1 Trap bakugan. When using the Big Game rules, you are only allowed 2 bakugan Traps. In order to play a bakugan Trap, you must have the same attribute as the bakugan in the battle that you want to use Trap with. Next you just lie Trap down on the gate card & when it opens, determine what is revealed, if it's an Attribute symbol, then your Bakugans Attribute is changed to that Attribute. If a G-Power is revealed, then your bakugan gains that amount of G-Power. After the battle, Trap goes to the used pile. When you are out of bakugans to use, take all of those in your used pile and move them to your un-used pile, you may also move Trap as well.
http://www.bakuganbay.com/Bakugan-Battle-Brawlers/Bakugan-Trap