If its a 6400/7400 Gateway branded server, check the floppy light. If its on solid during the no video/no post situation, you have a bad motherboard.
Otherwise, standard steps
- remove the memory and listen for beeps.
- try one memory module at a time (or pairs if dual-channel mem)
- remove all pci cards, and external usb devices. Disconnect keyboard and mouse
- clear NVRAM, usually with a jumper called CLR CMOS or a dip switch, check the back of the side/top panel.
- disconnect all devices, ide, floppy, scsi from the system board
- reseat power to the motherboard
- call their tech support. 877-485-1464 opt 2,3,3 - 24X7X365 and most servers have lifetime hardware phone support for troubleshooting just this type of thing.