First of all, I wouldn't generally advise anyone to give up on a US school on the first unsuccessful try (assuming that is the case). It seems awfully early in the game to be assuming that you will fail, although to be honest, a 3.1 sgpa is significantly below the average for any medical school.
Secondly, do not assume that a Master's Degree will make you a more attractive candidate. This web site and others are FILLED with threads regarding this issue. If you're still an undergrad, taking more undergrad courses can help build up that low gpa. If however you've already graduated that's generally not an option to you. I am no longer well versed in the intricacies of medical school admission so I'll leave that to others to weigh in on.
Bottom line is that if you are unsuccessful this year, I would give it another year before trying a Caribbean school. BTW, a 3.1 is also considered below the average for SGU students as well; you will have to have a pretty good MCAT score to obtain admission to SGU with that science gpa.
