In recent years, due to the technological development of various kinds of drones, also known as unmanned aerial vehicles (UAVs), drones have emerged as an effective solution for a wide area of applications. In such applications, employing multiple drones promises a better way to carry out a given mission than employing a single drone. To utilize multiple drones, all drones should establish a network that can provide connectivity among themselves. In this paper, we propose GCS-routing, a routing protocol for multiple drones employed by a ground control system (GCS). GCS-routing utilizes the information of drones that can be obtained from GCS, such as geographic information and per-drone flight schedules. The actual implementation of GCS-routing was applied to a fleet of drones, and evaluated in various aspects.