Here is a problem I thought would be easy, but it looks like it’s actually rather difficult, and Googling didn’t turn up any solutions. Suppose you have a document in Gimp with a bunch of text layers, each above another on the page like in a wedding invitation, and you want everything centered. First, set the text’s alignment within each text box to “centered.” That gets you part of the way there, by centering the text within each text layer, but the layers can still be off-center with respect to the page. So now choose the centering tool from the tool palette. It looks like a box with outward-pointing arrows on all four sides. Set the “Align Relative to” dropdown to “First item.” Then go to your image. Click on the background layer first. Then start shift-clicking on the text layers. (There must be no layers covering the text layers, even transparent or invisible ones.) Once all the text layers are selected, press the horizontal centering botton. It looks like this:
That should center all the text layers based on the background layer, without moving the background layer.