How to use gradient tool to combine two images ?

I tried to make a new sky to 360-image according to the instructions of these videos:

All parts of the videos were clear, but only the final point was unclear to me.
I selected “gradient tool” and then made straight line from the highest point of horizon to black area edge, but it only makes black in the area where I pull the gradient tool line.

Here is image, what kind of settings I used in gradient tool (should be same as in the video):

After adding the “layer mask” there is two small images on the same layer in the layer section. I selected the sky image in the layer section, so I do not selected the white small image in the layer section.

Why it makes black and not combine the panorama imge and the sky image together like in the video ?

I tried to do everything as in the video.
I even tried with same Gimp version than in the video, but same problem.

Thanks for answers and I can give more information if needed !

My guess is you have not clicked in the layer mask icon to make it active.
The active icon has a white border, not easily noticeable with a white layer mask.

Then it is as the video, gradient tool Black-to-Transparent and adjust it to suit on the canvas. There is also a small center slider for fine adjust ment.

40 second animation

Thanks for your answer !

It looks like that I did not select the layer mask icon.
I selected it too late before, now I selected the layer mask icon before I selected the gradient tool and then it works.

Here are some tips for Troubleshooting layers and masks. I personally have made and continue to make these mistakes, but very quickly realize my mistake and get back to working correctly with layers and masks.

Most problems working with layers and layer masks are caused by working on the wrong layer or the actual picture instead of the mask.

First check: Are you using the wrong paint colour for the mask?

If painting a mask to make visible the lower layer we need to be painting with black. To remove mistakes in a mask and conceal the lower layer we need to be using white. This can be verified by looking at which colour swatch is on top in the toolbox. The keyboard shortcut (x) reverses the foreground and background colours.

Second check: Which layer are you working on?

The layer that you are working on is shown in the layers dialogue by being highlighted in dark grey. Layer masks need to be applied to the upper layer to reveal the lower layer. The example to the right has the top layer active and the layer mask selected. If the bottom layer was selected any paint strokes would be applied to the image of the bottom layer.

Third check: Is the layer mask selected or the actual image?

After checking that the correct layer is selected, look for a white border surrounding the layer mask or the image thumbnail in the layers dialogue. The white border indicates which is actively receiving your instructions or brushstrokes. The example to the right shows that the top layer is active, but the layer mask is not selected because the picture thumbnail has been selected.

Fourth check: Are the relevant layers visible?

The eye symbol indicates that the layer is visible. By clicking off the eye symbol a layer becomes invisible. If the lower layer is invisible, you will not fully appreciate the effect of the layer mask as a checkerboard pattern is shown in place of the lower layer. The example to the right shows the top layer is active, the layer mask is selected but the lower layer is invisible.

Fifth check: Is layer mask accidently on bottom layer?

A layer mask on the bottom layer is used to create transparency but people often accidently put layer mask below the top visible layer and wonder why nothing appears to be working.

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Thanks for these tips !

I will keep in mind those tips when working with Gimp.