|Presumably this is a desktop/tower system? If so:|
Any display information at all; anything show on the screen (even if only briefly)?
When being stored and then "resurrected" it may be that a few internal connections may have become less firm (due to the manual handling of the unit); on-board plugs displaced, cards loosened etc.; also possibly one or more may cards require their edge connectors cleaning? Edge connectors can be cleaned with a soft pencil eraser, re-installed, then removed and re-installed (the removal/re-install a couple of times is to hopefully also clean the edge connector socket itself).
It may be the psu has suffered a little in storage, a capacitor or two may have decided to dry out/leak or otherwise fail. Do you have a voltmeter, and perhaps the manual for the PC? If so check the voltages around the system to see if they're as they ought to be?
When next you post back, actual model details the Gateway would be useful...
message edited by trvlr