If the opponent is just standing on the ground and you hit them with a G5 (I assume you mean the standard tap tap tap tap tap combo), then it should not use up your one ground bounce.
There are some circumstances where if you use a G5 on an already airborne opponent, one of the hits might drop them low enough to be considered a ground bounce.
Without knowing the circumstances you're talking about, it's kind hard to tell what you mean.