This page describes how to import a signature created by GNOME Keysign.
So you have received an email with your signed key.
Technically, the attachment is an encrypted OpenPGP key. That key carries a signature from the signer. So you need to decrypt the key first and then import it. A gpg pipeline could look like this:
gpg | gpg --import
Use your mail client to save the attachment somewhere, e.g. /tmp/key.pgp. Then open a terminal and type
cat /tmp/key.pgp | gpg | gpg --import
If everything works correctly, you should be able to read the lines:
gpg: encrypted with 4096-bit ELG-E key, ID AABBCCDD, created 2000-00-99 "Foo Bar <email@example.com>" gpg: key AABBCCDD: "Foo Bar <firstname.lastname@example.org>" 1 new signature gpg: Total number processed: 1 gpg: new signatures: 1 gpg: 3 marginal(s) needed, 1 complete(s) needed, PGP trust model gpg: depth: 0 valid: 1 signed: 1 trust: 0-, 0q, 0n, 0m, 0f, 1u gpg: depth: 1 valid: 1 signed: 1 trust: 1-, 0q, 0n, 0m, 0f, 0u