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].