If unfortunately Guitar Pro 8 closed before you had the opportunity to save your work in progress, there is a good chance you'll be able to recover a backup of your file as explained in this article.
Windows
Copy and paste the following address into Windows file explorer as illustrated below.
%LocalAppData%\Arobas Music\guitarpro8\
More than likely a backup of your file will be available from the "Autobackups" folder.
Make sure to remove the .bak extension in the file name to be able to open your file.
Keep .gp as extension.
Feel free to check the "backups" folder as well.
macOS
Open your Finder
Select the Go menu
Select Go to Folder
Copy and paste: ~/Library/Application Support/Arobas Music/guitarpro8
Check the backups and autobackups folders
Make sure to remove the .bak extension in the file name to be able to open your file.
Keep .gp as extension.