Here is a link to a forum. A poster says the battery is
"You have to take the laptop apart and remove the cmos battery. unless you want to cut a hole in the bottom. but if you do the battery is on the bottom side right under the vent next to the ram cover....
Electronic equipment repair and service manual search discussion :: View topic - schematic for gateway 400SD4 laptop

You can get a cop of the service manual here. It doesn't show the battery, only the top of the board, so I assume the forum post above is correct.
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