libgmp-dev 2:6.2.1+dfsg1-1ubuntu2 (riscv64 binary) in ubuntu lunar

 This development package provides the header files and the symbolic
 links to allow compilation and linking of programs that use the libraries
 provided in the libgmp10 package.
 .
 Both C and C++ bindings are present in this package. Package
 libstdc++-dev is required to use the C++ bindings.
 .
 The MPFR library (multiple-precision floating-point) used to be
 included, but has since been moved to package libmpfr-dev.