In scientific terms, "G" is used to measure gravity. "1G" is equal to earth's normal gravitational pull.Whether that applies here, I don't know.It could always be Jiggawatts!!
The bolded word is the answer. the italic sentence is my opinion.I remember that is from the Japanese official Bakugan website...EDIT: Finally find the source. It is from the Japanese Bakugan official blog: http://www.bakugan.jp/blog/2006/08/find the one on 2006-08-23 and the sentences below.'爆丸は、G(グラビティ)の高い爆丸をそろえてるね。''ゲートカードは、トランスフォームでG(グラビティ)の高さをいかして、ドラゴノイドのキャラクターカードでとどめを刺す作戦かな?'I don't know much Japanese, so you may make use of the translation website. I only know the bolded word is gravity.