We have tried numerous GAM update command to update a custom attribute via a csv file. We have used the following command. The field in Google Workspace is M365ImmutabelID. It is a text field.
gam csv M365.csv gam update M365ImmutableID "~M365"
gam csv M365.csv gam update M365ImmutableID '~M365'
gam csv M365.csv gam update M365ImmutableID ~M365
gam csv M365.csv gam update user M365ImmutableID "~M365"
gam csv M365.csv gam update user M365ImmutableID '~M365'
gam csv M365.csv gam update user M365ImmutableID ~M365
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