BuddyPress.org

I see that you’ve posted at the plugin’s forums at https://wordpress.org/support/plugin/bp-gtm-system and that’s the best place to post the issue.

Know that this is the holiday season so the possibility that the plugin will be updated within the next few days is low. If you’ve made a backup of your database and server files before you upgraded, I suggest reverting back to your old WP version install with those backups if you need GTM working.