Touring The Stars: Old Kentucky informs us that the world was targeted by the only use of a Gigaton class nuclear warhead in history. Though the main text doesn't go into detail about the exact yield, the later scenario hook gives us a number of two hundred gigatons. I'm... almost certain that this was a typo. But for fun, I decided to roll with it and plug that yield into the generic nuclear weapon rules found in Interstellar Operations.
Please note a few things. First, I don't think the interstellar operations rules were meant to be used like this. Second, the rules don't clarify what happens if the blast height exceeds the altitude of the Atmospheric Interface (because the rules aren't intended to accommodate a weapon where that's a possibility). I'm assuming the blast stops once there's no significant atmosphere to push, but if you want, you could extend the blast radius the full sixty five space hexes. I'm not as clear on the secondary effects. The radiation and electromagnetic pulse might not care.
Third, you can talk about how a pure fusion weapon in an airburst wouldn't produce a lot of radiation or fallout, but the blast radius is so large, I don't think you can avoid fallout no matter how high in the atmosphere you detonate it. We're talking about a blast effect over a thousand kilometers across here.
Fourth. No, there is virtually no way to use this warhead in a game beyond "bomb falls, EVERYTHING dies."
Yield: 200 Gigatons (200,000 megatons or 200,000,000 kilotons)
Base Impact Damage: 40,000,000,000 (400,000,000 capital)
Critical Hit Chance: 4+
Crater Depth: 66
Blast Radius (ground): 14,620 hexes
Blast Radius (air): 19,444 hexes
Blast Height: Interface (high altitude row 5)
Damage Degredation (ground): 2,735,978
Damage Degredation (air): 2,051,983
Secondary Effect Radius (ground): 29,240 hexes
Secondary Effect Radius (air): 78,070 hexes
Secondary Effect Height: 2,342,100 meters (high altitude row 130)
Weapon Mass: 200 tons (according to the text. Using the mechanics in Interstellar operations, such a weapon would weigh 100,000 tons assuming tech level E)