What gauge extension cord should I use for LED Christmas lights under 100W?

An 18-gauge cord is fine for LED lights under 100 watts. At 120 volts, 100 watts pulls about 0.8 amps, and a 50-foot 18-gauge cord is rated for around 10 amps, so you have massive headroom. Just make sure it’s outdoor-rated if used outside, don’t daisy-chain cords, and avoid running it under rugs or pinching it in windows.