I will also add in that the text tools in Gimp used to be a bit odd. Not sure about the 2.8 version but they were always hard to work with.

I honestly found it easier to create a flattenned image in GIMP and save it as a .png or .jpg, open it in Inkscape (another free product). Create a new layer in Inkscape and then create all the fancy textification you could ever want. Inkscape does a lot more for text than what's in Gimp. Then when you have what you need, export or just save that text only layer. Go back into gimp to your project and use the "Open as layer" option to bring in the text. So long as you keep same image size through the process it will be perfect.

Somewhere a long time ago I had this conversation when RobA helped to me to get this down. I'll have to check for a tutorial but I'm not sure if that exists.