How to revoke a GPG Key and upload in GPG server

It turns out that it is possible (and relatively simple) to delete and re-import the key, provided that it is on a keyserver (and provided that the revocation has not been sent to the keyserver, of course).

This is what I found to work (THEKEYID is the short ID of the key):

1. Delete the public key as follows (the --expert option allows the public key to be deleted whilst the private key is kept) :
   gpg --expert --delete-key THEKEYID

2. Confirm by pressing:
   y

3. Fetch the public key again from a keyserver:
   gpg --keyserver subkeys.pgp.net --recv-keys THEKEYID

Done!

Presumably this could also be done from a local (pre-revocation) backup of the public key, using gpg --import public.key instead of the third command.

Simply deleting the entire key (public and private) from the GPG Keychain Access GUI, and then restoring from a backup, did not work - I don't know why.

Do not use --gen-revoke, but --desig-revoke instead. From man gpg:

--desig-revoke name
       Generate a designated revocation certificate for a key. This allows a
       user (with the permission of the keyholder) to revoke someone else's key.

GnuPG will ask you whether you want to create a revocation certificate for this other key, for example revoking 0xdeadbeef with a key you're using:

$ gpg --desig-revoke 0xDEADBEEF

pub  1024R/DEADBEEF 2015-02-25 Alice

To be revoked by:

sec  2048R/E6F0D5F6 2015-02-25 Bob

Create a designated revocation certificate for this key? (y/N)

[...]

--desig-revoke will output the ascii-armored revocation certificate.