GMP is not a common system API like POSIX, so if you rely on it, you actually have to bundle GMP, or otherwise handle the dependency.
LGPL is fine for things like standard C and POSIX, because your program is not tied to that LGPL-ed implementation. The GPL itself recognizes "system library" as a special status. It is permissible for GNU programs to depend on proprietary libraries if they are system libraries. Under very similar reasoning, depending on LGPL-ed libraries is okay in an otherwise completely free program if those LGPL-ed libraries are system libraries.
LGPL limits you to dynamic linking, which is gross for this kind of thing. (Have you looked at a dynamic library function call lately?)
If you make a language depend on a LGPL library for numbers, that turns into GPL if you have to build a static image for an embedded system.
This is not a problem with malloc or printf which you just get from that system, rather than by carrying the GNU C Library into your program.
Legally you may be able to get away with adding a LGPLed library to a MIT or BSD program; in practice, licensing is not purely about "what can I legally get away with". What kind of hassles and restrictions will there be factors into it. Also philosophy factors into it: it's ideologically inappropriate for a liberally licensed (BSD, MIT) program to depend on special-purpose, non-system GNU libraries.
GMP is available under the LGPL.