Are a carbon fibre tripod heavy enough to carry a gimbal?

is the carbon or any carbon fibre tripod heavy and steady enough to carry the magnesium gimbal head in windy conditions and such? Or is it a better option to go with the aluminium tripod?

The relative weight difference between two otherwise nearly identical tripods is immaterial while you're shooting. With the weight of a gimbal, camera, presumably somewhat heavy lens, and especially the weight of your arms, the tripod weight difference will not be a factor.

Even when you're not shooting, the slight difference in weight won't really matter. If you're concerned, you should either add weight to the center column hook (many people like to use their camera bag, but in windy conditions, the bag and swing and hit the tripod legs). You can use bungee cords, ropes, bungee nets with rocks, and other methods to weigh down the tripod. I've seen people use tent stakes and bungee cords to "add weight" (downward tension), but because the cords are staked down, it doesn't come with the added downsides of free-hanging weight.

Specifically, those two tripods are both very capable, and will be plenty strong to hold a gimbal and long lens. I don't know if the aluminum TMA37AL comes with a short-center column replacement, but the TMA38CL does, and I'd certainly recommend using it. There's just no point using an extensible center column with a gimbal — the center column is single biggest contributor of vibration and instability on tripods.

Just in case you wanted more info, FStoppers favorably reviewed the TMA38CL, even compared to a 2-series Really Right Stuff tripod that was more than twice the Benro's price.