What happens if you don't set keymap "gg", "G", etc? Would they not work, or...? (Currently not on a *nix machine to be able to test it out, or else I wouldn't ask such a simple question :P)
"G" works with a number for recalling shell history. For example, "4G" recalls the fourth command in the shell history. However, "G" alone without a number does not work as it does in vi (jump to last line). I suppose that was the desired functionality.
It makes sense for "gg" to be absent, as it is not a vi command. It is a "vimism". The typical vi command for moving to the first line is "1G".
My guess is, that you redefine them to mean the semantically same in a new domain. What I mean is that gg and G lets you go to the top of the file and to the bottom of the file, right? But on the command-line, what is the top of your file? And what is the bottom?
I can only speak for the Emacs capabilities of readline: all commands which would usually change the line (previous/next-line, beginning/end-of-buffer) will use the history as the buffer. All of them preserve the line you are currently typing, which will be the end of the buffer.
It makes sense for "gg" to be absent, as it is not a vi command. It is a "vimism". The typical vi command for moving to the first line is "1G".