Just to set expectations on these gizmos and as I explain in the installation instructions included with the dampener, it will not completely eliminate gas gauge needle bounce - That is caused by the total lack of any fluid baffles inside of the tank to give an "average" fluid height for the float.
What it will do is reduce the amount of needle movement by half or so. The needle will still move, but it will be noticeably calmer than before. In order to get no movement you either need tank baffles (no can do on an existing tank) or an electronic sampling and averaging circuit that would cost a lot more.