One potential option for you would be to install Splunk Enterprise 6.5 on a second box, and set it up to act as a heavy forwarder. This being, a full Splunk Enterprise instance, whose purpose is to pull in data, such as by the GCP add-on, (and other apps, such as DBConnect), and forward it on to other Splunk instances in your environment.
I'm not certain from your question as to what your Splunk environment looks like, but even if you were just using an "all in one" instance, you could configure data inputs on that instance to receive forwarded data, and then configure the second instance to output data to your all in one box-more info on what that process looks like can be found here:
http://docs.splunk.com/Documentation/Splunk/7.0.1/Forwarding/Aboutforwardingandreceivingdata
Otherwise, if you have a distributed deployment (separate instances for search heads and indexers), you could simply point this new heavy forwarder to output data to your indexing tier. In either case, it would allow you to install the add-on on the last known supported version of Splunk Enterprise.
