The first hit is http://support.gateway.com/s/Mobile/Gateway/6000Series/5386nv.shtml

Two clicks and I get disassembly manuals. The battery is not easy to spot but it's the MA2A model like you find on Amazon or Ebay.
http://cgi.ebay.com.sg/Gateway-MX6124-MA2A-CMOS-Battery_W0QQitemZ280212397048QQihZ018QQcategoryZ31534QQcmdZViewItem

Remember this is one of those Pentium 4 Socket 478 based laptops so these have a bad history. Feel free to vent about it but to fix most I find I have to get a new mainboard.
Bob