In GIMP, how to get a selection from a mask?

Question

Let's say I paint a mask in GIMP and I want to convert that to a selection for applying filters, etc. What are the steps for doing this?

Using this sample image from Pixabay we'll create a mask for the purpose of adding more blur to the background.

Here's the original image:

Next I paint some white around the flowers on a new layer:

Then I add a layer below this one of solid black and merge the layers into a mask:

What is the next step for getting a selection from this mask layer so that I can apply a filter to only this area? Thanks.

[Note: This is a workflow I use regularly in Photoshop, but still have no idea how to do it in GIMP and would like to know how.]

Answer 1

If the mask layer is visible, open the Channels list, right click on any of the RGB layers and click Channel To Selection.

But instead of painting a layer to get the selection from it, you can directly paint the selection using the quick mask.