gmpxxll
A header only library that adds long long functions to GMP's C++ interface.
What you're looking for is in gmpxxll/.
Consult the documentation for usage details.
This repository exists so we could share the code between the various projects in the flatsurf project that almost all needed this functionality at some point.
This set of functions is certainly not complete and could likely be improved. Pull Requests are always welcome.
There are some tests and some benchmarks