OEM (Original Eqiupment Manufacturer) - The equipment that you recieve is the same as the original, just not in all the fancy retail packaging with a software bundle. For example, if you buy an oem HD, don't expect to get a pretty box(don't need another pretty box), a drive utility disk (can be downloaded from the internet), a manual (can be downloaded from the internet). Oem usually occurs when a company makes too many units and needs to sell them to make room for the new stuff coming out.
GXP - If your talking about the IBM Deskstar drives, it's just a series of drives that they produce, using similar technology to produce drives of varying sizes. Similar to a Quantum Fireball, Seagate Barracuda or Maxtor DiamondMax.
Hope this helps out.
Dick