I have a Gateway MP6954 laptop with factory 120GB

you need to check the specs on your gateway to be sure your laptop firmware can accept the 500G hard drive (might be too big for it to see). I would first check for any firmware updates from the gateway website first and any notes that they can provide to you.
Another point is the kind of hard drive you are looking at. Is it an SATA drive? PSATA? EIDE? This also needs to be clarified to be sure you have the right kind of drive. You can verifi\y that you system can see the drive by looking in the CMOS screens to see if it sees the drive. I suggest you start there and check for updates to the firmware if you do not see it in there (CMOS). Thanks!
mrstan