Not all PHP modules are loaded as separate modules and are part of the PHP base binary. In this case the module is loaded without a separate load statement/configuration.
If in addition a separate configuration file exists, e.g. /etc/php7/fpm/conf.d/gmp.ini, the web server tries to load the module twice which can cause these kind of messages.
# cat /etc/php7/fpm/conf.d/gmp.ini
extension=gmp.so