You can set every key to ultimate trust through opening the key edit command line
gpg --edit-key [key-id]
and running the
trust command. You will now be prompted to select the trust level:
Please decide how far you trust this user to correctly verify other users' keys
(by looking at passports, checking fingerprints from different sources, etc.)
1 = I don't know or won't say
2 = I do NOT trust
3 = I trust marginally
4 = I trust fully
5 = I trust ultimately
m = back to the main menu
5 will be the proper decision to achieve ultimate trust. Finally,
save to commit the changes and exit GnuPG. The same commands apply to both GnuPG 1.4 and GnuPG 2 (and newer).
Ultimate enables a key to introduce trust in the OpenPGP web of trust, with other words all ultimately trusted keys act as a starting point for trust paths. You should set your own keys to ultimate trust, but usually will not do so for other's.