The Goofball Proximity Surveyor, G.P.S. for short, is a special charm made by Mei Amanohokosaka.
Once attached to someone, the Goofball Proximity Surveyor allows Mei to see whatever the person it is attached to sees. After being assigned to help guard Touya Kagari, Mei places one on his head so she can track him. However, it was destroyed when Touya unleashed the power of the Origin of the Grimoire.
It also has another function. When attached to a Union it disables their Drive. Mei uses this to stop Acht's Veil Skin, allowing Mei and Kuon Glamred Stroheim to defeat her.