hello Sakkoch and welcome to SysChat
unfortunately i cant find any exact hardware manual/guide to opening your laptop's motherboard and replacing the cmos battery.
however, i found this picture:
its a picture of a Gateway M275 laptop motherboard. encircled in blue is the cmos battery.
Normally, the laptop motherboard can be uncovered by removing the laptop keyboard and palm rest. from there should be able to see the motherboard layout and spot the cmos battery
this is a link for removing the M275 laptop keyboard. the steps might be similar to your NV79 laptop. http://support.gateway.com/s/Manuals/Mobile/8509557.pdf
The Gateway website actually says that CMOS battery replacements/troubleshooting is best performed by their qualified technicians. i would guess they just want to spare users from the hassle of opening the laptop parts and exposing the motherboard.
If you are able to open the laptop and locate the cmos battery, you can take it out and bring to any computer store. see if they have a compatible battery type.
Goodluck, and feel free to let us know any updates on this