User blog comment:KnocknickKnock/9 gates + master of the celestial realm/@comment-24066070-20131208182752/@comment-7418318-20131213204517

When Hibiki downloaded the information directly into her mind, she probably knew every intricacy and thus could pull it off perfectly, but in the GMG she didn't have that luxury.

When she summoned Gemini the spirits even said that they havn't practiced enough and didn't even know if they could pull the spell of at all. With that it's only logical to assume that the spell was far from perfect and thus probably wasted a lot of energy, or maybe that the energy had to come from Lucy and Gemini, where the "perfect Urano Metria" might be able to gather the energy from elsewhere.