Whenever I try to use gpm I keep getting similar errors to this:
-bash-4.1# bin/gpm install pinpress

Preparing to install Pinpress [v1.0.0]
|- Downloading package... 100%
|- Checking destination... ok
|- Installing package... PHP Fatal error: Uncaught TypeError: Argument 1 passed to Whoops\Run::handleException() must be an instance of Exception, instance of Error given in /var/www/htdocs/mysite/vendor/filp/whoops/src/Whoops/Run.php:233
Stack trace:

0 [internal function]: Whoops\Run->handleException(Object(Error))

1 {main}

thrown in /var/www/htdocs/mysite/vendor/filp/whoops/src/Whoops/Run.php on line 233
Whoops\Exception\ErrorException: Uncaught TypeError: Argument 1 passed to Whoops\Run::handleException() must be an instance of Exception, instance of Error given in /var/www/htdocs/mysite/vendor/filp/whoops/src/Whoops/Run.php:233
Stack trace:

0 [internal function]: Whoops\Run->handleException(Object(Error))

1 {main}

thrown in file /var/www/htdocs/mysite/vendor/filp/whoops/src/Whoops/Run.php on line 233
Stack trace:

1. () /var/www/htdocs/mysite/vendor /filp/whoops/src/Whoops/Run.php:233

This example shows me trying to install the Pinpress theme but it happens with anything I try to install with gpm .

This is my first time installing/using gpm and your help is much appreciated. By the way, I am running PHP 7.0 on NGINX and using the conf file that is included with GRAV with a few minor tweaks specific to my system.

It's a PHP 7.0-related error. Stumbled on this today too, when an error happens in the front - instead of a more helpful error message.

Does not happen on 5.x. Could be a library issue. Inspecting.

Happened once today, but I cannot recreate it any more

Do you have APCu enabled?