Hi when checking with nitrokey.com/documentation/installation and navigating to Nitrokey HSM_GNU/Linux it is writtten:
Install OpenSC If you didn't do so already, change the default User PIN (default: 123456) and Admin PIN (default: 12345678) to your own choices. See these instructions. Generate new keys or import your existing RSA keys. Note that a Nitrokey initialized with OpenSC doesn't work with GnuPG/OpenPGP.*** But the other way around works fine.***
Alternatively, install this driver (source).
It looks to me that Nitrokey HSM does not work with gpg2 at all. So the above claim that it works the other way around is missleading. In addition I could not find any documentation about how to perform the other way around.