Let me guess, you used Netscape, right?

It used to have a habit of decompressing .gz files as it downloads them, without removing the extension. I'm guessing that they've not fixed it. This would explain why you didn't need the 'z' for tar.

And the dashes are optional in the tar in the empeg developer image, like somebody else already said.

I'll update the pages about the RW and RWM stuff, though.