What happens if a Country Member, whose FIAT currency I possess, has not yet opted to join the GGMF?

You will still be able to use the FIAT currency of a nation which has not yet opted to join, as it is still legal tender.

Once the Country Member joins, then the Account Holder must convert their FIAT into Gold Units before the end of the TRANSITION PERIOD or the Account Holder will forfeit the right to convert the FIAT at the end of the TRANSITION PERIOD.