.rar are compressed files similar to .zip, you need to download a program called winrar or winace to uncompress them. Go to www.winace.com or www.rarlab.com to get the files
A .gxb is a plain ARM9 executable
A .gxc/.gxe are files for commercial games and could be illegal cracks
A .fxe is an encrypted (I dunno why, and it's not to make sure someone doesn't read it) .gxb with icon & title & author info