For anyone looking in the future. Doing this with GCDS doesn't make sense as there isn't an appropriate existing attribute to sync from AD. Instead what I've done is this:
Schedule an export from AD that includes mail and sAMAccountName.
10 minutes after that export I run the following batch:
@ECHO OFF
gam csv C:\GamWorkingDir\usersAttrib.csv gam update user ~mail Enhanced_desktop_security.AD_accounts multivalued DOMAIN\~~sAMAccountName~~
DEL "C:\GamWorkingDir\usersAttrib.csv" /Q