Server Error Caused by Plugin

I am trying to update a website and I’ve just started getting this error (code 500) as of two days ago. I can not save changes to the site unless I disable this plugin and then of course the spaces where I have used this plugin breaks. Here are the error details that WordPress has sent to me.

Error Details
=============
An error of type E_ERROR was caused in line 431 of the file
plugins/gt3-photo-video-gallery-pro/plugin.php. Error message: Uncaught Error: Class ‘gt3pg_pro_plugin_updater’ not found in /plugins/gt3-photo-video-gallery-pro/plugin.php:431
Stack trace:
#0 /wp-includes/class-wp-hook.php(287): gt3_photo_video_galery_pro->cron_callback()
#1 /wp-includes/class-wp-hook.php(311): WP_Hook->apply_filters(”, Array)
#2 /wp-includes/plugin.php(544): WP_Hook->do_action(Array)
#3 /wp-cron.php(138): do_action_ref_array(‘gt3pg_pro_cron’, Array)
#4 {main}
thrown

How can I resolve it?

The page I need help with: [log in to see the link]