In practice, Person A uses GPG to encrypt messages or data so that only Person B can read/use them. This is done by using a public key which essentially says that "I am who I say I am."

You need a copy of the public key for the person you want to secure the message/data for. This is conceptually similar to needing a person's phone number before you can call them, or a person's email address before you can send them email.

From Wikipedia:

GnuPG is a hybrid-encryption software program because it uses a combination of conventional symmetric-key cryptography for speed, and public-key cryptography for ease of secure key exchange, typically by using the recipient's public key to encrypt a session key which is only used once. This mode of operation is part of the OpenPGP standard and has been part of PGP from its first version. […]

GnuPG encrypts messages using asymmetric key pairs individually generated by GnuPG users. The resulting public keys may be exchanged with other users in a variety of ways, such as Internet key servers. They must always be exchanged carefully to prevent identity spoofing by corrupting public key ↔ "owner" identity correspondences. It is also possible to add a cryptographic digital signature to a message, so the message integrity and sender can be verified, if a particular correspondence relied upon has not been corrupted.

# make a key (if you don't already have one)
gpg --gen-key
 
... RSA and RSA is okay ...
... 2048 bits is okay ...
... 1y is okay (but choose your own duration) ...
... Real name: First Last ...
... Email address: me@email.com ...
 
# lookie in here
cd ~/.gnugpg
 
# generate an ASCII output of your public key
gpg --armor --output first_last.asc --export 'First Last'

• Installing GPG Tools Suite for macOS
• Keybase and KBFS