Will a galvanized tin roof act as a ground for a magnetic rooftop antenna
I take it that you are talking about a cellular reception antenna. if so, then yes, depending on the specific make of antenna, the design is to ground through the metal to enhance the reception. 


The metal acts as a ground-plane , which acts as a secondary antenna, using this technique should vastly improve performance. 
The larger the metal surface, the better the performance
The device will work fine with no metal surface, however you should notice a performance degradation on a non-metal surface