With a magnet and calipers you could have a guess-free answer.

 

Very true. But unfortunately, I'm out of town and not at the house. Thus, the problem...

 

Hard to say what the roof in the picture is, but 20 gauge was common for the original roof on historic homes 1820's to 1900.

 

Looks to be a more modern standing seam, I'd say 24 gauge.

 

Is this a serious question? AI? Spam? How is anyone supposed to give a definitive answer based on a picture. And if everyone who responded said 24 ga steel … would you believe them and sign the contract without checking further?

 

No one in his right mind would use aluminum. Larry is right, 24 gauge is probably most common nowadays. Everything is cheaper / thinner.