First, open a Terminal and run gpg with the --gen-key ("generate key") parameter.
gpg --gen-keyYou will be given a choice.
Please select what kind of key you want:
(1) RSA and RSA (default)
(2) DSA and Elgamal
(3) DSA (sign only)
(4) RSA (sign only)
Enter 4. Next, it will ask for the size of your key. Default is 2048, but I suggest 4096. Now it will ask you for the expiration date. You don't have to set one, but it's good to have, if you plan on creating a new key at some point, maybe because of increased security standards like keylengths of 6144 or different key formats or ciphers. That way people are forced to check for a newer key.
You have to enter a name, e-mail and optionally a comment. Confirm the data input with o and enter your passphrase twice. Your key will now be generated.
You will see something like:
gpg: key 968328E7 marked as ultimately trusted
public and secret key created and signed.
gpg: checking the trustdb
gpg: 3 marginal(s) needed, 1 complete(s) needed, PGP trust model
gpg: depth: 0 valid: 7 signed: 0 trust: 0-, 0q, 0n, 0m, 0f, 7u
gpg: next trustdb check due at 2018-08-11
pub 4096R/968328E7 2014-03-25
Key fingerprint = 9F72 21B4 671F F9B1 D6E3 CEA6 17E8 DFDF 9683 28E7
uid Sāfto Rangen <firstname.lastname@example.org>
It's time to export your key!
gpg -a --export 968328E7 > 968328E7.asc
gpg -a --export-secret-keys 968328E7 >> 968328E7.asc
This will create a combined public/secret keyfile called 968328E7.asc which you can use with Retroshare.