Marktkauf is right on the border. The tram stop going there is in Germany, the tram stop coming back is in Switzerland.
A few weeks ago the rules change, and the GA is no longer valid once the tram leaves Switzerland. So you either need to buy a ticket for one stop, or just risk it on the way out. On the way back your GA is fine.
I don't know what type of ticket you need to buy as I've never had a GA. For people without a GA it's a 2-zone ticket (half-tax valid); you need something called a Connecting Ticket for Fr.2.70. If you were going right into Weil, the price would be the same but on the way back you'd need to buy a one-zone RVL ticket to get you back to the border where your GA would then take over.
http://www.bvb.ch/docs/default-sourc...8.pdf?sfvrsn=2