How to enable GMP Extension
How to enable the required extensions with commandline access
1. Via Commandline:
sudo apt-get install libgmp-dev php7.0-gmp curl php7.0-curl
2. Add the line below to the php.ini (e.g. /etc/php7.0/apache2/php.ini) [?]
extension=gmp.so
3. Restart your webserver (e.g. Apache):
sudo service apache2 reload