When I edit an existing key and add a new user identity, then this identity is first shown as having "unknown" trust. Therefore, I select this new uid and "trust" it. This change is not visible until I exit the (text) user interface, though.
Steps to reproduce:
(Get some key pair, gpg --gen-key)
# gpg --edit-key KEYID gpg (GnuPG) 2.1.20; Copyright (C) 2017 Free Software Foundation, Inc. This is free software: you are free to change and redistribute it. There is NO WARRANTY, to the extent permitted by law. Secret key is available. sec rsa2048/NUMBERSANDLETTERS created: 2017-11-19 expires: 2019-11-19 usage: SC trust: ultimate validity: ultimate ssb rsa2048/MORENUMBERSANDLETTERS created: 2017-11-19 expires: 2019-11-19 usage: E [ultimate] (1). Some Name <some@e.mail> gpg> adduid Real name: Unknown Email address: dont@mail.me Comment: You selected this USER-ID: "Unknown <dont@mail.me>" Change (N)ame, (C)omment, (E)mail or (O)kay/(Q)uit? O sec rsa2048/NUMBERSANDLETTERS created: 2017-11-19 expires: 2019-11-19 usage: SC trust: ultimate validity: ultimate ssb rsa2048/MORENUMBERSANDLETTERS created: 2017-11-19 expires: 2019-11-19 usage: E [ultimate] (1) Some Name <some@e.mail> [ unknown] (2). Unknown <dont@mail.me> gpg> uid 2 sec rsa2048/NUMBERSANDLETTERS created: 2017-11-19 expires: 2019-11-19 usage: SC trust: ultimate validity: ultimate ssb rsa2048/MORENUMBERSANDLETTERS created: 2017-11-19 expires: 2019-11-19 usage: E [ultimate] (1) Some Name <some@e.mail> [ unknown] (2)* Unknown <dont@mail.me> gpg> trust sec rsa2048/NUMBERSANDLETTERS created: 2017-11-19 expires: 2019-11-19 usage: SC trust: ultimate validity: ultimate ssb rsa2048/MORENUMBERSANDLETTERS created: 2017-11-19 expires: 2019-11-19 usage: E [ultimate] (1) Some Name <some@e.mail> [ unknown] (2)* Unknown <dont@mail.me> 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 Your decision? 5 Do you really want to set this key to ultimate trust? (y/N) Y sec rsa2048/NUMBERSANDLETTERS created: 2017-11-19 expires: 2019-11-19 usage: SC trust: ultimate validity: ultimate ssb rsa2048/MORENUMBERSANDLETTERS created: 2017-11-19 expires: 2019-11-19 usage: E [ultimate] (1) Some Name <some@e.mail> [ unknown] (2)* Unknown <dont@mail.me> gpg>
Still shows [ unknown] instead of [ultimate].
If I then hit CTRL-D (EOF) I get Save changes? (y/N) which I answer Y and am back at my shell prompt. If I then run gpg --edit-key KEYID again both uids are shown with trust [ultimate].