Gimp 2.10.6 Issues While Following a Tutorial

[MistyBeee]

I'm not a Gimp user, only Photoshop and I don't know the tutorial you're mentionning, but it looks like a normal comportment to me : if the Mountains layer is set in Multiply, it needs a background to be visible because it's the interaction between the multiply blend mode and the background which matters here. So if you hide the background layer, probably the mountains multiply mode doesn't have any "support" to blend with and looks just invisible while trying to blend with... nothing. It doesn't means there is a problem with the mountains layer, nor with Gimp
To understand better what the Multiply mode does, have a try with just two layers : your mountains, for example, and a background you change : you'll see an empty, pure white or pure black backgrounds will show nothing, while any colored or textured backgrounds will change the mountains in different ways.

[KCRileyGyer]

Oh, and the tutorial is listed on this site in the tutorials that are pdfs sticky.