Galeb Duhr vs Gary Mitchell: who would win?

Galeb Duhr favored — Slight edge (~55%)

Galeb Duhr · Dungeons & Dragons · Base

Tier
9-A, up to 8-C
Attack Potency
Small Building Level+, up to Building Level
Speed
Hypersonic+, up to High Hypersonic+
Durability
Small Building Level+, up to Building Level

Gary Mitchell · Star Trek · Base

Tier
8-C, up to HIGH 6-A
Attack Potency
Athlete Level, up to Multi-Continent Level
Speed
Athletic Human, up to Massively Hypersonic+
Durability
Building Level, up to Multi-Continent Level

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