Hi mcwmoo ,
if you want to consume rest api of third party system within clarity process via gel script, you can go through GEL Script consuming CA PPM REST API and if you are looking to get started for CA PPM Rest API then follow CA PPM REST API link for information.
Besides this I will suggest, a staging table which could be a CA PPM Custom object for data flow and handling, however we won’t have rest api for ca ppm custom object, so you might need to use xog for achieving that.
I hope this give you some information for your integration.
Regards,
Prashank Singh