I set up a unique url for each CS member and asked them to make the Gartner review ask at the the end of each of their QBRs every quarter. I didn't get a ton that way but I got a few. You should also be targeting high NPS scores. My company sent out an NPS survey every quarter through Pendo and sent me anyone who gave us a NPS 10. I then emailed those customers asking for a GPI review. I got a lot of reviews that way.