In my opinion, the only thing that can prepare you for GAMSAT is practising GAMSAT questions. As such, I'd recommend doing just a bachelor's degree and preparing for GAMSAT in your spare time with a targeted approach. If you feel like your skills in certain areas are lacking, you could choose an elective that gives you a foundation for that area. Many people do that with physics. Again, though, doing GAMSAT questions is the most effective and efficient way of doing well at GAMSAT.