I have a gateway PC for the last three years and about a month ago my LCD monitor had no signal, I tried it with another PC and it works perfectly. I thought it was the video card so when I opened the PC I notice the motherboard has it integrated, my bro told me to install a PCI Express but still nothing so I also changed the CMOS battery and change the RAM but nothing. Can someone help me with this??

The motherboard itself may be dead. Do fans and everything else power up?

Things to try:

Pull the power cord and let it sit for awhile and then plug it back in and attempt boot.
Unplug all the drives and adapter cards and attempt boot with a minimum hardware configuration.
After three years, my prime suspects would be hard drive and power supply.

Yes, I also forgot to mention the hard drive don't seem to be working either but the fans are. I think is not even trying to do the POST. I will try doing what you say and let see what happens.
Thanks!

As the machine is a couple of years old, see if you can try just a known working PCI video card to see if that would work. As mentioned if the video card works in another machine and just not yours I would suspect the mother board itself also