Hello Zebotoph71
From what I know, leaving the country is not an etraordinary case in which you can cancel the contract during its minimum contract term. The GA Travel Card GTCs do not foresee such a possibility.
But, the SBB Community is not the SBB customer service. In order to get a final response from SBB, you may contact the SBB Contact Center by phone: 0848 44 66 88 (CHF 0.08 / min, waiting times are possible). Or see an agent at the SBB Travel Centre.
Kind regards,
Roland