Have you tried using a different monitor or swapping video cards? Are there beeping noices when you turn the PC on? Have you tried calling Gateway? I hear Gateway tech support is pretty good, but my friend use to have a Gateway PC and he hated their tech support.
Anyway, if you are desperate, you can try the following. Do this as a LAST result if you still can not fix the computer. It is going to sound stupid, but if you are desperate, I assume you will try anything. I assume your Gateway PC and your dad's PC both have ATX motherboards. If both power supply on both the PCs are close in watt, you can try taking your dad's power supply and put it onto your motherboard. Make sure when you pull the plugs out, you know where to plug them back in. Have a piece of paper and pen ready to write down what went where.
1. Unplug your dad's PC.
2. Ground yourself.
3. Unscrew the screws that holds the power supply to the case of your dad's PC. There should only be 3 or 4 of those on the back of the case.
4. On his motherboard, there should be a bunch of wires that come out to white connection that plugs to the motherboard. Press the little white clip and pull out the connection. Unplug all the other connections that connect to the HD, CDROM, etc..
5. On the Gateway motherboard, ground yourself and unplug that same white connection that connects the power supply to the motherboard.
6. Plug your dad's power supply to the Gateway motherboard. Do not worry about plugging in the connections to the HD, CDROM and other stuff. You just want power going through the board to see if it has enough juice for the motherboard and video card.
Hope it works, but if it does not, sorry for wasting your time. This actually happened to me about 6 years ago when I was working for big company. The CFO's PC would look like it would boot up, but nothing would show on the monitor. There would be no beeping noices. I swapped everything with a computer similiar to his. I got so desperate, I called my manager, told him what I did and he suggested I try swapping the power supply. I told him the lights are on and I do not think it is the power supply. He told me to do it, so I did. What do you think happened? The damn thing booted up fine. My manager said there was enough juice to light up the LED lights, but not anything else. The power supply had gone bad. So I hope this will help. Remember, do this as a LAST resort. I don't want you frying your PC and blaming me for it, hehe.