There is too little info in your question. All I can do is guess.
Perhaps it's not charging because the battery is already fully charged?
Perhaps the battery is defective?
Perhaps there is a problem with the battery connectors?
How long does your laptop operate when using a fully charged battery? Have you been periodically (say once a month at least) been discharging the battery?
it gives the percentage of what is charged and right now it's continually decreasing because it's not working.
Well, it sounds as though the battery may be dying though I'm still just guessing. I tried searching the Gateway site for your laptop but came up empty. Perhaps you would have better luck by visiting Gateway's support page and using the serial number on your laptop.
If you can bring up the Gateway support page for your notebook then look for battery related information. In particular look for something about battery conditioning or maintenance.
Since I couldn't find anything to go on at Gateway's site I fell back to what I am somewhat familiar with, my MacBook. Below is an excerpt of Apple's recommendation for their batteries. I assume your Gateway notebook also uses a lithium-based battery so the advice should also work in your case.
If it was me, then before replacing the battery I would try to completely discharge/recharge it for at least a few cycles to see if that helped.
For proper maintenance of a lithium-based battery, it’s important to keep the electrons in it moving occasionally. Apple does not recommend leaving your portable plugged in all the time. An ideal use would be a commuter who uses her MacBook Pro on the train, then plugs it in at the office to charge. This keeps the battery juices flowing. If on the other hand, you use a desktop computer at work, and save a notebook for infrequent travel, Apple recommends charging and discharging its battery at least once per month.