Just experienced the same issue: apt purge php-gmagick
followed by systemctl restart apache2 solved that issue Debian 9.6 ii apache2 2.4.25-3+deb9u6 amd64 Apache HTTP Server ii libapache2-mod-php7.0 7.0.30-0+deb9u1 amd64 server-side, HTML-embedded scripting language (Apache 2 module) ii php-imagick 3.4.3~rc2-2 amd64 Provides a wrapper to the ImageMagick library removed this one: ii php-gmagick 2.0.4~rc1+1.1.7~rc3-1 amd64 Provides a wrapper to the GraphicsMagick library
