Gateway is probably correct when they tell you the limits of the present motherboard. So there's really no need to worry about what it is.
You should probably consider getting a new case and power supply along with a barebones system that you can then add in your existing drives. Gateway uses proprietary power supplies and sometimes cases so new components may or may not fit and the power supply probably won't run them.
That said, look for a barebones system that includes case, power supply, motherboard, processor, RAM, keyboard, mouse, etc.
As for the motherboard, you may have to go with something that has onboard video and sound, unless your present system has these separate cards already.
Here's a couple of good places to start:
This space reserved for highly intelligent observations and witty remarks.
Effort & Courage are not enough without Purpose and Direction.
---- Auther Unknown