G5 will be full body Armament Haki and he will be like he ate the Mera Mera no mi, as he will be on fire instead of steam, I also blv its the last Gear and being on fire should do it, when steam becomes too hot it can somehow create fire, so yea fire punches should do, combination of Armament haki and Fire, Luffy gears are the once to make him more stronger than most opponents, and if he gains his awakening it should be the worst kind to the GURA GURA no mi, as he may change the whole landscape to rubber and Earthquake won't be effective to it.
His awakened state should also create punches that are on par with KING KONG punches, using the environment to his advantage, Luffy Gomo Gomo No mi, may be the only Devil Fruit to stop Earthquakes if the sorrounding can turn to Rubber, I also blv Blackbeard stole that power so Luffy may be the only one to be able to defeat him, at marineford it looked like no one was able to counter whitebeard devil fruit, so luffy might be the only one to counter that devil fruit and once BB finds that out he will want to kill LUFFY before he awaken his true power.