These are just power stats for the two.
First up, we have Dark Gaia. A manifestation of darkness and an hyper-energy organism.
Destructive capacity: Planet level+ as Perfect Dark Gaia
Attacking strength/potency: Same as DC.
Speed: Not too fast, can't even move around though he has supersonic to hypersonic reactions. (reacted to a weakened Super Sonic)
Durability: Possibly multi-planet+ (when Super Sonic knocked his blasts back at him all it did was stun him for a few seconds)
Intelligence: Not to smart, animal-like though he was smart enough to create a shield that would protect himself from harm, so I guess he's average in the smarts department.
Strength: Dark Gaia can tear planets apart with his bare fucking hands. A class strength isn't necessary.
Weakness: Though very durable, Dark Gaia's eyes are highly sensitive to damage for Sonic in his base form was able to inflict significant damage with just a few homing attacks.
Abilities: Possession, control over darkness, super strength, can grow in size up to that of a large continent in his perfect form, can absorb dark energy at will, telekinesis, energy manipulation (can fire massive laser canons that could pierce through planets and could summon energy balls that could flat out blow up a planet), can summon spawns (though this weakens him for they are all part of his essence), draws power from the night, can heal, immortality.
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Now, for the Demon Shredder. I'll be covering his base form and his dragon form.
Destructive capacity: Multi-City+ in base form, potentially planet level in dragon form. (He was capable of reality warping the planet, and he has control over the mystical energies of the entire planet so it seems like something to think about)
Attacking strength: Same as DC in said form
Speed: Supersonic to hypersonic travel speed via powerscaling, teleportation, relativistic reactions
Durability: Seems to vary depending on his concentration. One moment a laser beam composed of light energy pierces right through him (which he regenerated from), and another moment he tanks multiple blasts capable of city busting and it doesn't even tickle him. I assume he has planet level durability at the best.
Intelligence: Above average, mostly relies on his power but he is an alright strategist.
Strength: Class 1000+ (caught a punch from Silver Sentury and threw him into orbit who was capable of causing multi-city block sized explosions and throwing giant robots into the Sun with little effort.) Can amp his strength with dark magic.
Weakness: Can be over-confident at times, always boasting about how powerful he is and plays around for his own amusement. Disposing of his helmet and gauntlet would break his immortality, and getting another version of the Shredder to meditate for a while would weaken him.
Abilities: Dark magic, teleportation, regeneration, immortality, reality warping (which increases his power with every passsing second), mind rape, telekinesis, control over the elements, super strength, super speed, can grow in size up to the size of a large city, matter manipulation, relativistic reactions, can drain light/dark energy, shape shifting, healing, can become intangible, mastery in all the martial arts, can summmon armies of demons and monsters, control over nature (caused a solar eclipse with his mere existence alone), can fire energy blasts capable of blowing up multiple cities to potentially blowing up a planet, contains planetary attacking potency, nigh-invulnerability,