Try this...Better check my work as I just through this pictorial together quick for a quick, dirty, and easy remote switch to throw unit into pinpoint, all metal fixed, or all metal track (depending on what you have that switch set at on the GT). What you'd do is cut the right post halfway between the switch and the circuit board using a dremel or small snips (toe clippers work well). Bend these two ends away from each other a bit to insure they will never make direct contact again. Now solder a wire from the switch side here to a jack you install on the box. Another lead from jack would go to the post end on the circuit board side where you cut it. When soldering these protect the circuit board below the switch with paper or something in case you drop some solder. Try to work quick and solder only to the very cut end of both to avoid excess heat transfer to the switch or board. If your not real experienced with soldering remember flux works wonders to clean/prep the solder conection.
Anyway, the jack will be normaly closed (n/c) when there is no plug installed in it so the circuit and GT switch will work as normal without the remote switch plugged in. Once you plug in the plug for the remote switch the jack will go to being an open circuit (n/o). The circuit is routed through the jack down the wire and to the remote pinpoint switch. The circuit will stop there unless the switch is closed. Several types of switches can be used here. The type that won't close unless constantly held down, the type that will close or open with every push (staying in that mode until pushed again), or you could even use a normal toggle switch such as Whites use if you like them trigger style. I'd probably opt for a waterproof button style that will stay open or closed until pressed again to switch and hold modes.
Here's whats happening...With no plug GT switch works as normal. With remote switch plugged into GT switch for disc iron off and disc iron on will work as normal (but with no ability to use the remote switch to throw it into all metal unless remote switch is closed). If you throw the GT into all metal and the switch isn't close the GT will still think it's in disc iron on mode. This is the way you want to hunt, with the GT set at all metal. With remote switch open the GT will operated in disc iron mask on mode. Press the remote switch and it's now in all metal mode. Depending on where all metal mode switch is set (pinpoint, fixed, or track), the GT will now go into that mode of operation. Press the remote switch again and you're back to disc iron on. For this reason this remote switch can be useful for doing reverse discrimination even if you don't need pinpoint. Hunt while in all metal tracked or fixed and then press the button to quickly check the target in discrimination.
Later I'll wire this thing up and make sure it's working. I'll dig up the part #s from Radio Shack and throw up a real schematic with the proper wiring of the jack, plug, and switch including any other details needed. Pictures as well. This is just a way to cheat the GT into doing something without getting more complex. For instance, you could have a remote switch that would have all 3 operating modes, bypassing the GTs. More complex then needed for most people, though. How many people need to switch from disc iron off to disc iron on and all metal remotely. Most use disc iron on and all metal (pinpoint) modes.