That GG1 is an older model with a an electronic reversing unit and horn. The DCS Explorer is only compatible with Protosound 2 & 3 engines that were manufactured 2000 or later. This GG1 came out in 1998.
The reason you only hear the horn is because the you are using a DC power supply to operate the explorer. When the engine sees power from the DC supply, it sees the DC offset caused by a horn button on a transformer and continuously blows the horn.
