How to make GnuPG display full 8-byte/64-bit key ID?

Alternatively you can use:

gpg --keyid-format LONG -k 0xDEADBEEF

Or:

gpg --keyid-format 0xLONG -k 0xDEADBEEF

You can see the long key ID using the option --with-colons (yes, very intuitive).

To print only the long key ID, use something like:

$ gpg --list-keys --with-colons XXXXXXXX | awk -F: '/^pub:/ { print $5 }'

Just to point out a sanely named option to remember,

GnuPG 2.2.13 on macOS Catalina 10.15.4, --list-signatures option displays the key ID as well:

$ gpg --list-signatures

gpg --version                              

# gpg (GnuPG) 2.2.19
# libgcrypt 1.8.5

To get the Short KeyId of the existing keys. In the example below there is only 1 key and the short keyid is 567C9ABC, displayed after rsa4096

gpg --list-keys --keyid-format SHORT

#~/.gnupg/pubring.kbx
#pub   rsa4096/567C9ABC 2021-03-06 [SC]

The Long KeyId = 32FA0BE4567C9ABC

gpg --list-keys --keyid-format LONG
#pub   rsa4096/32FA0BE4567C9ABC 2021-03-06 [SC]

EDIT: the keyId could be obtained using either gpg --list-keys or gpg --list-secret-keys