64 bytes are usually more than enough buffer for the serials. If you're getting the GPS data at 10Hz and every line is perhaps 100bytes, the processor has to process 1kB per second which is not much even for an Arduino (which is ways slower than today's PCs).
In almost every case if you loose characters on the hardware serial interfaces you made a mistake in your sketch, like having delay() calls in it. If you do your programming correctly you won't have problems loosing characters. Show us your (complete) code and we try to find the software problem.