If there were arithmetic operators that were not overloaded, Perl would invoke numeric conversion before using the builtin arithmetic operations, but I expect Math::GMP to overload all of them, so int may be the only built-in that actually invokes the numeric conversion.
I expect (but am not certain) that the '0+' overload is also invoked if an XSUB attempts to convert an object to an IV or UV. This is probably (but I have not traced the code) also how builtin arithmetic operations (which are not used on Math::GMP objects) would end up invoking it.