Depends on what is wrong. Is it the DC Power Jack? The jack you plug the AC adapter (Charger) into?
Average cost is around $100 for a DC Power Jack replacement. (It's labor intensive, but the jack itself usually runs around $8 to $14 )
Most computer 'repair' shops do not have the skills required to perform this procedure.
The easy way is to just replace the motherboard. (Also more Ka-ching in the pocket)
Just a typical DC Power Jack replacement,http://www.laptoprepair101.com/laptop/2007/12/06/dc-power-jack-repair-guide/
Disassembling a Gateway MX 6000 series laptop,http://www.insidemylaptop.com/disassemble-gateway-mx-model-ma2a-series-laptop
Some designs of the motherboard, have the DC Power Jack mounted in an area on the motherboard, that DC Power Jack failure is a reoccurring failure.
The motherboard cracks in the area the DC Power Jack is mounted. (Breaking the circuit traces)
In this instance it's better to replace the motherboard. Of course it's going to fail later on.
Your question: "Can you replace the motherboard?"
I have confidence in you, I believe you can.
Doesn't take a rocket scientist, but does require patience.
1) Remove ALL Power to the laptop. AC adapter (Charger), and Battery.
2) It would behoove you to buy, and wear an ESD wrist strap. (Electro Static Discharge)
Connect the alligator clip to a good ground source.
I connect to the metal frame of an empty desktop computer case.
(Average cost is around $3 to $8. Look online for reference. Visit computer repair shops, and stores like Radio Shack for purchase)
Your body carries Static electricity. Static will fry out (Short Circuit) the hardware components inside a computer. Especially a Processor.
With wearing an ESD wrist strap, and connecting to a good ground source, your laptop is safe.
3) There may be as many as 60+ screws. There may be 12, or more different types of screws. DO NOT mix them up! Use a multi-compartment container, and label the compartments for the area the screws came out of.
You can use a clean egg container, (Or two) in a pinch. label each egg holder.
4) You may want to take photos, and make notes as you disassemble. Makes a real handy guide for reassembling. Also nice to have if you get interrupted in the middle.
Obtaining a motherboard:
You might check Ebay. Sounds like advertising, but it isn't. Just one of my sources.
Also check Laptops for Parts on Ebay.
Some sell a laptop that has a broken LCD screen as the only fault.
Post questions in a Comment.
This is what the motherboard looks like, (Gateway Support),http://support.gateway.com/s/Mobile/Q106/BladeK8F/4001199R/4001199Rmv.shtml