It's definitely just a standard lots-of-bits. Every bit of information is needed, lose one and the whole test will be wrong. For current wavefront exponents, like 2^60,000,000-1, save files are around 8 MB, or ~60M bits -- exactly what you'd expect.

Also Prime95 doesn't use GMP, its code is all hand-coded assembly, optimized specifically for the LL test and x86 architecture, written by one very dedicated George Woltman.