The problem is you are not running PHP 5.5+ in the CLI… Try:
> php -v
This will tell you which PHP you are running for things such as bin/gpm. You probably will need to contact your hosting provider to ask them where a newer PHP is located on the server. They can probably also help you add that new path to your account’s path configuration so it will get picked up rather than the default version.
Your default php version is 5.4.45. Your Hoster might have changed the php version your vhost/site runs on to 5.6.27 or whatever, but your default version still is 5.4.
You have to prefix bin/gpm with the path of the php 5.6.x executable.
E.g. on my CentOS VPS the default php version is 5.4.16, I installed php 7.1 via remi repo and I’m executing gpm like this:
/opt/remi/php71/root/bin/php bin/gpm
Of course you can alias the path or, like I did, make links in /usr/bin to be able to user this shorter command:
php71 bin/gpm
So, you’ll need the path of where php 5.6 is installed on your server to prefix the gpm command.