It is a multiple-precision arithmetic library: https://gmplib.org/
GMP is currently required, however, the version used by Julia (>6) is dual-licensed under GPL v2 and LGPL v3 (from link above):
Since version 6, GMP is distributed under the dual licenses, GNU LGPL v3 and GNU GPL v2. These licenses make the library free to use, share, and improve, and allow you to pass on the result. The GNU licenses give freedoms, but also set firm restrictions on the use with non-free programs.
According to the Free Software Foundation’s interpretation of LGPL, this means that you can use it in a proprietary program as long as users can re-link their own modified version (and you redistribute any GMP-specific source changes you make, if any). The simplest way to allow this is to use dynamic linking, as Julia already does with GMP by default. See:
(as always, if this could materially impact your business: seek competent counsel)