Well, it’s been a rough day. One that started off with simple changing spark plugs ended with fishing for a socket and two magnets. Socket fell down through the hole and onto the piston, tried to follow it with two strong magnets that are now attached to the cylinder wall. What do I do now?
I think your biggest problem is, you're using the wrong type of magnet. You need to use one which is permanently attached to something. It is typically called a "Mechanic Pickup Tool". You'll need this to get the socket out. You can get them from Amazon or from Harbor Freight, or probably any hardware store.
To extract the socket, run the piston up into the cylinder about 1/2 way.
Warning: Do not push the piston too far up or you will most likely damage the piston, valves, and/or head in the process.
Ensure the socket is in a place you can see it. Then run the pickup tool through the sparkplug hole and extract it. When selecting a pickup tool, you'll want to get one strong enough to hold the socket strongly so it will want to come out with the tool. You man also need to use a long, thin screwdriver to get it away from the magnets so it will be attracted to the tool only.
Next, you need to get the permanent magnets out. For these, you'll want to use a thin screwdriver. The problem you'll have is, the magnets will want to stick to the cylinder wall which I believe will be cast iron. To get them out, push them away from the cylinder wall with the screwdriver. Be careful not to scratch the cylinderwall in the process. Once the magnet is away from the cylinder wall it should be sticking to the screwdriver. As long as it does not come back into contact with the cylinder wall, it should be an easy extraction from that point. Do this with both magnets and you should be home free.
If you are unable to get them out using these methods, your only other choice is to have the car towed to a shop to have the cylinder head removed/replaced. I believe the 2000 GT had the 4.6L engine, which is a SOHC design. If you haven't done work like this before, it is something you'll probably want someone knowledgeable doing, because if you get it wrong (timing the valvetrain), you run the risk of complete destruction of the engine.