

I haven’t come across a single tutorial that ‘unlocks’ GIMP for new users (if I’m reading your question correctly) but I’ve found Davies Media Design to be an excellent resource for video tutorials, both free and paid. Also Bernard Hooft has provided a free video course and book.
Be aware that lots of learning resources are being updated after the 3.0 release, so some tutorials may no longer be valid.
Would ‘overwrite with zeroes’ in gnome Disks work?