How do I open the case on a gateway (GM) MFATXNIN NMZ 300L ?

There's a security screw on the back that needs to be removed to allow the front latch to release the side panel. It's the stupidest most confusing case ever. The latch is very hard to see, it's part of the black plastic in the front, just push down on things until you see one move a bit. It's sort of like a car door handle movement.

Try this link for various user manuals. Once you find the right one, it should tell you how under the "hardware" category.

http://support.gateway.com/support/manlib/Desktops/desktop.shtml

you might need to open the front cover somehow then four screw or look for two buttons at the back (somewhere I don't remember) where you push them together at once to open.