GMP core php extension Formula was removed
Last version was
php54-gmp requires the following formulae to be installed:
autoconf 2.69 Automatic configure script builder
gmp 6.1.2_2 GNU multiple precision arithmetic library

Recent formula history

Mike McQuaid Deprecate tap.
ilovezfs php54-gmp: bottle revision to rebuild.
Andy Blyler Revert "php 5.5.34"
Andy Blyler php 5.5.34
Andy Blyler cleanup extension testing

Formula code at GitHub