Despite being undead, Life spells and items have no effect on Gil Turtle; it is instead weak to Ice. Its attacks include Earthquake, which does heavy damage unless Float is cast, and Turtle, which hits two party members at one turn and can inflict Confuse, similar to the Shield Dragon's Knock Silly. It will also cast Earthquake when defeated.
A good strategy is to cast Float before the battle to dodge Gil Turtle's Earthquake spell at the end of battle. Once the battle starts, the player should immediately cast Blink, which will help dodge Gil Turtle's attacks, as well as summoningGolem as backup, in case Blink wears off before the party has time to recast it. The player should cast Blink and Golem regularly, while using the Bard's Requiem Song, which harms the undead, and Blizzaga (especially when used with the Spellblade command).
Another strategy consists of having a party composed of one Knight and three Bards. The Knight should wear Bone Mail and an Elven Mantle to boost evasion, and ensure the Bards can use Summon and Time Magic between them, so Golem can be summoned and Return can be cast if needed. The Bards should be brought down to critical HP before facing the Gil Turtle, so the Knight will automatically Guard them while they use Requiem.