Using GIMP, I'd like to use the HEAL tool to remove some text from an image. How can I isolate the modifications made by the heal tool to a separate layer, so that I can easily toggle the text on/off after it is removed?
The Healing Tool isn't really designed for removing text - especially large text. It might work OK with small/thin text, but I think it's better for removing small spots/imperfections.
You would probably be better using the Clone tool, then you can set the target on one layer, then use it on an another layer directly.
You can't easily do that with the Healing Tool as far as I'm aware. You'd have to jump through some hoops, for example: make a selection first, choose the target, do the healing, copy it, then undo all changes, then Edit > Paste as > New Layer in Place.
Another possibility is to duplicate the entire layer (or do Layer > New from Visible), and work on the duplicate layer. Then you can toggle that layer on/off. Afterwards, I suppose you could also surround the deleted text area with a selection and add a layer mask if you need to.
Another possibility is to use GIMP's Resynthesizer plugin - and the Heal Selection plug-in. Link to get them is here. It's similar to Photoshop's content aware fill. You could then make a rough selection around the text, run the filter, copy it, undo once, and finally paste as new layer in place.