Here's a comprehensive guide for defeating the Demon Gate in "Final Fantasy 7 Rebirth," including requirements, a detailed walkthrough, strategies, and additional tips:
Requirements:

Demon Gate Boss Guide in Final Fantasy 7 Rebirth
•Location: Inside the Temple of the Ancients during Chapter 13.
•Rewards: Experience, AP, and story progression.Party Composition: Cloud, Tifa, and Aerith for this fight in Chapter 13, "Where Angels Fear to Tread."
Materia:
•Fire, Ice, Lightning, and Wind Materia: Demon Gate has no specific elemental weakness but can use elemental attacks which you can counter.
•Healing Materia: Cure or Pray for Aerith to manage party health.
•Barrier Materia: To reduce damage from the boss's attacks, especially when the arena shrinks.
•Assess Materia: To learn about Demon Gate's weaknesses and attack patterns.
Gear:
•Equip Cloud with the Hardedge for damage or Twin Stinger for Counterstance; Tifa with Mythril Claws for stagger; Aerith with Guard Stick for magic.
•Consider gear with high defense or resistance to elemental damage since Demon Gate uses various elements.
•Level: Aim for around level 50-55, but with strategic play, lower levels can work if you manage your resources well.
Detailed Walkthrough: 
Phase 1: Initial Attacks:

Demon Gate Boss Guide in Final Fantasy 7 Rebirth
1.Cave-In, Petrifeye, Inexorable Orb, Repulsion, Crushing Grip
2.Strategy:
3.Avoid Cave-In: Move out of the glowing squares on the ground to avoid being crushed.
4.Dodge Petrifeye: Stay mobile to avoid the beam attack.
5.Inexorable Orb: Dodge or block these; they home in but are slow.
6.Repulsion: Position yourself away from the wall when this is about to happen.
7.Crushing Grip: Free the grabbed character by attacking the hand.
Phase 2 (Encroach - Arena Shrinking):

Demon Gate Boss Guide in Final Fantasy 7 Rebirth
1.Encroach: The arena starts to shrink, making dodging harder.
2.Strategy:
3.Interrupt Encroach: Use strong abilities or spells to stop the walls from moving in. Fire, Ice, Lightning, or Wind spells work if they match the elemental attack it's about to use.
4.Focus on Damage: Use this phase to deal damage, especially with Tifa for stagger.
Phase 3 (Second Demon Gate at 50% HP):

Demon Gate Boss Guide in Final Fantasy 7 Rebirth
1.Double Trouble: A second Demon Gate appears, doubling the attacks but also dividing your attention.
2.Strategy:
3.Focus One Gate: Concentrate on destroying one Demon Gate at a time to simplify the fight.
4.Arcane Eruption: Watch for the element they're charging and counter with the opposite element to stop it.
5.Beckoning: Move away from the gravitational pull to avoid being dragged in.
Strategies: 

Demon Gate Boss Guide in Final Fantasy 7 Rebirth
•No Elemental Weakness: Use a mix of elemental spells to counter their attacks or interrupt Encroach.
•Stagger Management: Pressure by stopping Encroach or by hitting them with the opposite element during Arcane Eruption. Tifa is key here for stagger.
•Defensive Play: Use Barrier frequently, especially as the arena shrinks. Manawall can help against Arcane Eruption.
•Synergy Abilities: Use these to gain ATB quickly or to deal with both gates simultaneously when they're close.
Tips: 

Demon Gate Boss Guide in Final Fantasy 7 Rebirth
•ATB Management: Use Tifa to build ATB with her combos, save for big attacks or to interrupt Encroach.
•MP Management: On Hard Mode, use MP sparingly; consider MP Absorption Materia.
•Positioning: Constant movement is key, especially as the arena shrinks, to dodge attacks.
•Limit Breaks: Save for when both gates are staggered or one is almost defeated for maximum impact.
•Hard Mode: Equip Elemental Materia with each element on different characters' weapons for versatility.
•Summoning: If you have a summon like Bahamut, use it when both gates are staggered for massive area damage.
•Aerith's Role: Critical for healing, use her Arcane Ward for doubled spell effects when possible.