Try opening the .xcf in a text editor and searching for "font". This will take a long time to load for anything more than a very simple image.
I made a quick gimp project with Krupper as my font and then opened the xcf in text editor and this is what i found:
(font "GimpFont"
(fonthash "3f73e56f9b4eed8091b647543d766cfac650ff79cf05dcede9f4f25b0f3fdaec")
(fullname "Krupper (Plain):001.001")
(family "Krupper")
(style "(Plain)")
(psname "Krupper")
(index 0)
(weight 100)
(slant 0)
(width 100)
(fontversion 65536))
(font-size 45)
(font-size-unit pixels)
(antialias yes)
(language "en-us")
(base-direction ltr)
