Are you building LFS or CLFS and which version?
Which book are you using?
Which chapter is giving you troubles?
You mention CLFS and I do see a BUILD64 variable, but there is no CLFS 6.3 (version 1.1 is the stable CLFS atm).
I also don't find any mention of gmp in either of the 3 CLFS docs (32 Bit Builds / Intel/AMD x86
or 64 Bit Builds / x86_64
or Multilib Builds / x86_64
) and I'm assuming you don't have a powerpc.
I do find a mention of gmp in LFS 6.3, but that's chapter 6 not in 'section 5', but that's not x86_64 related.....
Could you give us some more info please?