Open systemsettings5->Online Accounts and add a "Google" account.
It displays a name for the new account now:
{F8110424}
(see the screenshot in the bug report for how it looked before)
Note that you may need https://cgit.kde.org/kaccounts-integration.git/commit/?id=2420b04e3d01cd02c26e238221a7603614830d15 in kaccounts-integration, as the "Online Accounts" is more or less broken in 19.12, see https://bugs.kde.org/415267 (although creating an account should actually work).
Open "Network"->"Google Drive" in dolphin (you need to have kio-gdrive installed of course), the new account(s) is/are shown now and can be accessed:
{F8110429}
Also tried with a patched signon-ui that returns some fixed username, it used that one then instead of the fallback, as intended.