GJ Events has no form of payment integration. I suppose you could use By URL Part protection in CBSubs and protect the attend URLs though, which are as follows.
option=com_comprofiler&task=pluginclass&plugin=cbgroupjive&action=plugin&func=events_yes&cat=CATEGORY_ID_HERE&grp=GROUP_ID_HERE&id=EVENT_ID_HERE
To protect all attend URLs remove the id check and it should work for all attend URLs.