Problem booting up

check the ram.change the slot and disconnect the hardrive before that

It seems that your first boot was on cd or dvd, or may be your hard drive did not detect, to check both of them go to BIOS set up by pressing F2 or Delete on your keyboard to enter set up. When you are in set up mode go to Boot menu (if you can not see boot menu go to advance). In a boot menu select HDD as a first boot and Cd or DVD as a second boot, if you can not find your HDD it means that your HDD did not detect, and you need to replace it with a new one.
Hope it gives you a solution

Do you get the 12v side also?

You've got a failed backlight - requires a new inverter assy - not an easy component to replace.

Does the Gateway appear to boot up but you do not get an image on the screen?
Does the power LED light up and does the hard disk LED flash as the Gateway boots up?
If so then the Gateway could have a back-light problem or the video card may be faulty.
To determine the exact fault, please click on this link and follow my instructions to determine the fault an possible fix :-
http://www.fixya.com/support/r3598095-faulty_laptop_lcd_screen

red light is stands for hardware boot and the green one is stands for system boot and disc boot. when red light remains solid it means your system is in 0% cpu usage and nothing doing, stop working

So you've checked the supply, but you didn't replace it, right? Sounds to me like that's your culprit. It's not able to supply the current to run the processor (the biggest draw) and the output is dropping too low to run the system. A new supply should get you running again.

Try booting up with windows disk in the drive

dead

The flashing LED is because your system is in "sleep" mode. By default windows Vista puts your computer in sleep. What this means is it doesn't shutdown the power completely it leaves a little going through certain computer components (more specifically the ram) to speed up the boot time when the power button is pressed again.

In XP a the default shutdown scheme is a complete shutdown where power is turned off after the shutdown process is done. If you are using XP you are putting your computer to sleep when shutting down.

As for the boot problem. Newer motherboards have an LED on the actual board to indicate that it is receiving power. If it is of either the power supply switch (at the back of the case) if off or the motherboard connection is loose and the shaking makes it fall into place. (Shaking is usually not necessary and indicates a problem...)

There are a few possibilities I can deduct from your description:

• the connection from your power supply to your motherboard isn't properly seated and is causing this
• the power supply switch (not the case switch) is being  turned off and results in the computer taking a few seconds before the internal LED become solid
• ... the dreaded sign that the power supply is failing and is on it's last legs...
  

My suggestion is to check all connections and if that doesn't work borrow a power supply from a friend and swap them to see if it boots properly.

(If a friends with a spare power supply isn't an option a local PC repair shop should be able to help you out... but probably for a fee...)