gpg2: no secret key

…of course gpg2 can't find the secret key because it's looking in the wrong file.

That's not the only file it's looking at.

In GnuPG 1.x (and 2.0), the "secring" used to have a duplicate copy of your keyblock's public data as well, so it was entirely self-contained (and the only difference between gpg -k and gpg -K was which file it'd read), but at the same time more difficult for the program to maintain.

In GnuPG 2.1, secret keys are now stored independently – they're maintained by gpg-agent, which keeps them in ~/.gnupg/private-keys-v1.d/. So both gpg -k and gpg -K now have to read the OpenPGP information from the pubring, but the latter additionally asks gpg-agent about which certificates have associated secret keys. If you're using strace, you should notice a connect() call right after reading the pubring.

If GnuPG didn't automatically migrate the keys, just import the whole secring directly:

gpg2 --import ~/.gnupg/secring.gpg

To check the agent's contents manually:

$ gpg-connect-agent 
> keyinfo --list
S KEYINFO 926145FFCA32B3E6E079A0CF73EA77C40733A349 D - - - P - - -
S KEYINFO BACFB81EAFC864F4AB2926E8B1F55AD579F78D1A D - - - P - - -
S KEYINFO FF3D1DD51B9C79E148CCCEA5F7F3E25EC96048B7 D - - - P - - -
S KEYINFO 4D29EF1460F164CDB11D0FC0247214660ACDD60F D - - - P - - -
S KEYINFO 06B13685B9AA429B9CABCE480930D74B991C8DF0 D - - - P - - -
S KEYINFO B28DB8D045654E8A6A40466A07FCD9E432935E29 D - - - P - - -
OK
> /bye
$ 

These are "keygrips" – compare them with GnuPG's secring:

$ gpg --list-secret-keys --with-keygrip
/home/fred/.gnupg/pubring.kbx
--------------------------------
sec   ed25519 2018-08-18 [SC]
      2357E133AD5D24F6CB2C1B0CEF4F7ED27E252632
      Keygrip = 4D29EF1460F164CDB11D0FC0247214660ACDD60F 
uid           [ultimate] Fred Foobar <[email protected]>