Mew vs Gaea: who would win?

Gaea favored — Slight edge (~55%)

Mew · Pokémon · Base

Tier

5-B, up to HIGH 3-A

Attack Potency

Planet Level, up to High Universe Level

Speed

Massively Ftl+

Durability

Planet Level, up to High Universe Level

Gaea · Percy Jackson · True Form

Tier

5-B, up to HIGH 4-C

Attack Potency

Planet Level, up to Large Star Level

Speed

Nigh-Omnipresent

Durability

Planet Level, up to Large Star Level

A heuristic estimate from normalized VS Battles Wiki stats: tier, attack potency, speed and durability.