You've given the ability to create custom attributes (1st image) when this has been done, that custom attribute is not exportable via the download options (image 3) for all users.
The custom attributes needs to be something that can be mapped and used for provisioning Google SSO.
You can do this with the free, open source, command-line, indispensable GAM tool. Check out:
for more details.
Specifically, gam print users customschemas all should do the trick, per https://github.com/taers232c/GAMADV-XTD3/wiki/Users#print-user-details
Hope that helps,
Ian