I have text cutting out pieces (letters) from background layer using alpha to selection on the text and using that selection as mask for the background layer (like in this question how can i make alphabet like this from any image in GIMP?).
Is it possible to use the text layer as mask for another layer directly so that any changes are immediately reflected in the result?
Any other way to have the (modifiable) text as mask, perhaps some special mode of layer blending?
I am using Gimp 2.10.
You guessed right.
Use blending mode Split or Erase inside a layer group to make holes to the generally visible image (=a gradient fill in this example). The background can be seen through the holes. Edit the text to change the holes.
The dotted rectangle is GIMP's indicator for the size of the selected layer. It can be switched off in the View menu.
Be sure the blending mode of the layer group is = Normal.
You can add more layers into the group under the TEXT. They all will be punched.
Unfortunately there isn't a way to have an editable text layer as a layer mask directly in GIMP. You need to use Alpha to Selection to make the selection in order to make the mask. However what you can do, is retain the editable text layer if you ever need to make changes, so that you can easily create a new mask. You can simply hide the text layer by clicking the eye icon next to the text layer in the layers panel. Then save the document as an XCF, for future edits.
You could certainly use layer blending modes, but it wouldn't be an actual layer mask as such. Also some experimentation would be required I think. Offhand, and without seeing how the rest of the layers are constructed, I couldn't just say use such and such a blending mode.
However here's an example where it could work. I have typed the layer modes in the layer descriptions so you can see them.
