Make sure you have turned on backup to Google Drive in Notability. If not click here.
Tap the download button in Notability.
Select Google Drive from the options.
Look for a folder called Notability or individual notes in your Google Drive.
Each folder of notes must be imported separately. Create a Subject for each existing folder by selecting the + (1) at the top of the screen and creating a Subject (2).
Open the newly created folder for your subject and select Import All (1).
Go through all the folders from the Notability folder in your Google Drive and import your notes into the corresponding folders.
When you have imported all your notes select Done (2).
Your notes will be back in Notability filed by subject.