In Logic 7, there is a "Learn MIDI" button in the Key Commands window. This was a great feature to assign a specific function to a MIDI command (a MIDI note for example). This could be the cause of your issue.
Open the Key Commands window (Logic Pro > Preferences > Key Commands...) and hit your G4 key. If this key is assigned to a command, it will appear in the window. If it is the case, click the "Learn MIDI" button to activate it and select the function and hit "Backspace" on your keyboard to delete it.
Tell me if it works!