This was posted by the CEO of Fanatec on their website regarding the demo and the CSL Elite PS4 wheel:
"Some of you might have tried the demo version of GT Sport which is online as of today.
Unfortunately, at this time it is not possible to select the CSL Elite Racing Wheel in the GT Sport controller options menu. Therefore players can’t enjoy some of the wheels advanced features in the game, such as Force Feedback.
We apologize for any inconvenience caused, and are working hard with our engineers and partners with the aim of rectifying this for the games launch. We will continue to update you and appreciate your patience. "