By default, every note is attached to one page and Google Mail is actually only one page. But you can still attach notes to only certain emails by following these steps:
1. You have to enable the option "Update notes when anchor changes" in the FloatNotes Preferences -> Locations.
2. Now you should be able to click on the gear of a note and select "This page... including anchor". This will make the note appear with the email only. If you have a close look at the address bar when you have a email open, you should see something like `#inbox/12ed7f748e5ec6b9` at the end of the address. Everything after the # sign belongs to the "anchor". Therefore, by selecting the option "..including anchor" for a note, it only appears if exactly this anchor is present.
You have to do this manually for each note but I plan to include the possibility to specify the default location for websites individually.
I hope this helps!
Best,
Felix