You can do this by using the attach to placed object button. Place your sign near the Gantry, and with the sign selected, goto the Tracking tab and select attach to placed object. This button then turns orange and you can select your Gantry. The sign will attach itself to the Gantry, and then you can use the relative position options to position the sign on the Gantry.
Now when you move the Gantry, the sign(s) move with it.
I should add that the “attach to placed object” button can be found in the Position Controller Rollout in DVSP Explorer when the sign is selected (see attached).