I really, REALLY should not have spent time on this right now with all I have to do, but damn it! I'm so excited! And it's so helpful!
I do hope this is all easy to read and clear, sorry it's so large, but there is much I felt needed to be shared to get a lot of the potential Gradient Masks have!
As said, I will take Q/A to fill in blanks and change the file later on to reflect these gray areas. Enjoy, and I would LOVE to see what people do with this new knowledge!

EDIT:
HOLY COW! I did NOT expect this to get so popular and loved in just 3 days!

Thank you all! I'm so happy that this is something you all can find useful and learn from. All of the comments so far too; it's all very encouraging, and I hope to share more hidden tips and tricks to the power of art

It's just fantastic!
One thing I'd like to add is that instead of using a layer mask, you could just create a clipping mask with the gradient map on top
Thanks!