The only requirement for being able to sync your notes with Gmail is that your iPhone be updated to iOS 4. When you set up your Gmail account on your iPhone, you will now have the option to sync your notes.
If you have several Gmail account set up on your iPhone, you will be able to sync your notes to specific accounts. In the top left corner of the Notes.app, a new button will appear letting you choose what account you want to sync with.
One downside to this is that it’s a one-way sync, which means that you can only sync your notes from your iPhone to your Gmail account. You can’t sync from Gmail notes to your iPhone.