Don`t quote me on this, But i`m pretty sure that this would not work.
The reason: Lightguns, The type that are/were used on the likes of the Nes, Master System, MegaDrive et all. Work in conjunction with the tv frequency/refresh rate eg. 50hz/60hz to detect where abouts the controller is pointing when the trigger is pressed.
LCD`s, Like the one in the GP32, Laptops and pretty much all flat panel displays work in a completely different way. They do not work on frequencies/refresh rates like a standard tv, But response time. I am in no way an electrical technician, But am pretty sure that what you are suggesting is not technically possible. Someone correct me if i am wrong.
BTW: This was discussed in a thread a while ago. But i can`t be assed to find a link.
Trooper