When you see E=Fb, that is what's called an 'enharmonic equivalent.' Gbb (called G double-flat) would technically be F, not E. Here's how to figure it out: Go one half step down from G. It would be F# or Gb. Now go one more. You're at F natural, or E#. Think of it this way: one half step down from F is E. One flat sign means you go down a half step from the natural note. So Fb would be a half step down from F, which is E. Similarly, going up one half step from E natural would be F, so E# is F.
Enharmonic equivalent means that you have multiple names for the same pitch. Eb and D# are enharmonic equivalents because they are the same pitch, just different names. We use these different names because of the key we are writing in. So for example, if you are in Bb Major, you would call the pitch an Eb, because Bb Major uses flats. However, if you are in the key of E Major, you would call the pitch D#, because the key of E Major uses sharps.
You would see something like Gbb if you were in a key that already has a Gb in it, like Db Major, Gb Major, or Cb Major. I hope this helps!
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