I don't know why you got that bak file. I've never gotten that before but I know that you only need the gcm file to play the game
I don't know why, if you ripped it yourself, you would have these problems. However, if you got the game off a file-sharing network, most likely the "game.gcm" is an image without the "garbage data" allowing it to take up far less space with some games. And "game.bak" is included just in case the other smaller version doesn't work. And if it didn't work, you simply rename the "game.bak" to "game.gcm."
Here is a tool to stream it the easiest and newest version. http://home.arcor.de/sarxer/cbs-p245.rar Also don't burn it. It will not effect speed of the game playing back. Only burn if you dont have enough space on your harddrive.