From ultimarc's ArcadeVGA FAQ:
* Do Light Guns Work?
The Act-Labs USB gun works but there are some factors to bear in mind. The gun will only work if the H and V sync are not connected together. The H and V signals must both be connected to the monitor. OR, if a J-PAC is used for the connection, this will also work. A few people have found that ATI cards (including the ArcadeVGA) used with certain models of the D9200 monitor, produce a signal which is slightly too low for the gun. The Act-Labs gun seems particularly sensitive to the VGA signal levels and ATI cards do generally produce lower signal levels than other cards. We believe that some older variants of the D9200 load the signals (ie reduce the levels) more than others.