A) Replacement LCD screen backlight could be bad.
Happens. Sometimes you get a bad part. (Bad LCD screen assembly, in this case)
LCD screen may not be the correct one.http://support.gateway.com/s/Mobile/2008/GodzillaFX/2528410R/2528410Rsp2.shtmlhttp://support.gateway.com/s/Mobile/2008/Godzilla/2528407R/2528407Rmv.shtml
A screen Inverter, and an LCD screen; are a MATCHED unit.
B) Screen Inverter:
May be bad, or the Wrong one
C) You may not have the screen Inverter, connected correctly to the Backlight.
Connections aren't tight.
D) The Display Cable TO LCD screen connection may not be tight, or correctly connected.
I know the display shows graphics, but a poor connection of Display Cable to LCD screen, can make the Backlight dim out.
I'm betting it is a poor connection, or the wrong screen Inverter, or wrong LCD screen.
(Be shame to kick that laptop to the curb. Nice laptop)