Gmax Finale heals 1/6 of total HP, not 1/2.
Max of 344 HP, healed from 263 to 320 HP, which is 57 HP. It's consistently healed that regardless of foe or amount of foes HP.
Another time healed 115 -> 172, again 57 HP. This was through a Protect as well.
344 / 57 = 6.03, confirming the 1/6th...