Unless you're confident about scoring in the top percentiles of the UCAT, I would not do it. UCAT cutoffs for graduates tend to be much MUCH higher whereas the GAMSAT cutoffs are more obtainable. In a previous year, I got 7th percentile UCAT and was rejected pre-interview by 3. With the GAMSAT, I was given 3 interviews, 2 offers and 1 waitlist. You may consider doing both, but you have to focus on ensuring you give each enough preparation time, GAMSAT requiring the most.