As far as I can tell, there's no way to make a macro to use a keyboard shortcut. Those are only intended to be used, well, with a keyboard. If you've configured it to G, you can call your chocobo simply by hitting "G" on the keyboard, or if you're using the "chat-specific keyboard" option, you'll have to use CTRL+G.

If you're using a gamepad, you can easily call your chocobo from the micro-menu, you should have a key configured to access that (as in, you really should... if you don't already, go set one in the game config)

I don't see what putting mount/dismount in a macro would allow you to do that you can't already do with fewer keystrokes right now.

At any rate, there are other threads already requesting the addition of /mount and /dismount commands.