The latest version of this package is 9 years old and only mentions supporting "PHP 7.0.1+". If you need ImageMagick support you should use Imagick instead.
Information for installing this PECL extension may be found in the manual chapter titled Installation of PECL extensions. Additional information such as new releases, downloads, source files, maintainer information, and a CHANGELOG, can be located here: » https://pecl.php.net/package/gmagick.
Note: The official name of this extension is gmagick.
The latest version of this package is 9 years old and only mentions supporting "PHP 7.0.1+". If you need ImageMagick support you should use Imagick instead.