Where do the iPod’s notes go when they’re synchronized with the Gmail account

There's a difference between syncing with iTunes and over the air sync.

If you set up your Gmail account to use over the air sync then when you open up the Notes app (on the iPod), you should notice a back button on the top left that allows you to change which account you use for notes. If you choose your Gmail account and start creating notes, you will start seeing them appear in your Gmail account, under the "Notes" label. This sync happens automatically once you press the "done" button on each note (as long as you have an internet connection).

This has nothing to do with iTunes sync.

You will only see notes that you create under this account in Gmail. You will not see all of your old notes. For that, see this question.

The problem is that Google's implementation of IMAP isn't really 100% IMAP, so it doesn't really support notes. I've been trying to get it to work for quite a while which ended up with me losing all my notes (although they may still be in the Google Mail archive somewhere).

I've actually given up trying to sync notes between my Mac/iPhone/Google and have gone for the Plain Text app which stores the notes on DropBox, this seems to work so much better. Now if only I could delete the Notes app from my iPhone.