Hi
If you take the IRE train from Basel Badischer Bahnhof or from Schaffhausen to Bad Säckingen, the GA travelcard is not valid on this route.
If you are travelling from Schaffhausen, then your GA is valid until Erzingen. If you are travelling from Basel SBB then your GA travelcard is valid until Basel Badischer Bahnhof.
Please note that the tickets for the IRE can not be bought on the website sbb.ch
Please use the website bahn.de or a ticket machine at the train station between Basel Badischer Bahnhof and Schaffhausen.
You can check here the map of validity of your GA travelcard: https://sbb-tickets.cc/en/tickets-offers/travelcards/ga-travelcard/ga-travelcard-area-validity.html
Kind regards
Alban