Hi Luciano,
1. the existing code is functional and adequate. GMP offers no new functionality and may offer performance improvements or may not. But the existing code is capable of being tuned also.
2. The existing plugin is internal; with the "new" linkage I wrote two years ago now internal plugins have a much simpler call interface into the VM (no double calls through a pointer anymore; just direct calls on the VM functions). But LGPL forces us to make the plugin external. Hence using GMP implies a loss of performance in marshaling arguments and allocating and returning results.
IMO these two together make GMP irrelevant. Less compelling reasons are
3. Yet another library to install on all build environments; AFAIA it's not present on non-Linux platforms
4. Being a library we don't control we're dependent on others fixing it.
What reasons, other than brand identity are there for using GMP?