How old is the laptop? In my experience laptop batteries have a lifespan of about 5 years under average use. As they get older they will loose capacity.
You can get a new battery from Gateway but they are expensive so try to get the old one tested first as said. Some normal electronic component stores may test it too.
If you do get a new battery I would recommend getting a genuine gateway charger too - You could break your brand new battery by using a slightly incompatable charger!
Cameron Gray