security – How can I verify a signature of the sender of the email?

I received an email that includes a PGP signature such as .asc.

From what I have understood, it guarantees that the email is really from the sender and that it is possible to decrypt the signature with its public key.

I try to use gpg to do that. But the documentation says that I will also need the document for which the signature should be verified. So I can not find a command example for what I'm trying to do.
I read for example this documentation https://www.gnupg.org/gph/en/manual/x135.html

Have I mixed something?
If I really need to deliver the email content to the command as well, it will be difficult to decide which parts of the email I will need and in what format I will have to assign it to the gpg invocation.