Search for the PHP version

brew search gmp

homebrew/php/php53-gmp
homebrew/php/php54-gmp
homebrew/php/php55-gmp
homebrew/php/php56-gmp
homebrew/php/php70-gmp
homebrew/php/php71-gmp

brew install homebrew/php/php71-gmp

Find the GMP config file

ls /usr/local/etc/php/7.1/conf.d

cat /usr/local/etc/php/7.1/conf.d/ext-gmp.ini

[gmp]
extension="/usr/local/opt/php71-gmp/gmp.so"

Edit php.ini

nano /Applications/MAMP/bin/php/php7.1.1/conf/php.ini

[gmp]
extension="/usr/local/opt/php71-gmp/gmp.so"

Restart MAMP!

Add the PATH variabele to your profile:

nano ~/.bash_profile

source ~/.profile
export MAMP_PHP=/Applications/MAMP/bin/php/php7.1.1/bin
export PATH="$MAMP_PHP:$PATH"
alias php='/Applications/MAMP/bin/php/php7.1.1/bin/php'

Test

php -m | grep gmp