How to install GMP for PHP7 on Ubuntu
To install GMP for PHP7.0 on Ubuntu, run:
sudo apt-get install php7.0-gmp
Make sure your php.ini contains the following:
extension=php_gmp.so
To find out where your php.ini is located, run:
php --ini
For PHP 7.2:
sudo apt-get install php7.2-gmp
The new way in PHP 7 to install extensions is to just write "php-EXTENSION" In this case it's as simple as:
sudo apt-get install php-gmp