So after some further brainstorming yesterday with a college of mine, we decided to pass a simple query string parameter along with the redirect. The JS code we wrote to be included on the pages hosted on the Gartner server will then read the query string and if the parameters are correct we create a cookie from their server.
So then subsequent pages can just check for the cookie and this time be able to find it since the cookie was created on their server
Just thought I would update this with a solution since someone else might come across "cross domain" cookie doom