Not done with GIMP, but with G'MIC (command-line version), but still I think I have to share this.
Zonderr is one expert G'MIC user and he has done some magic there for aligning faces photographs :

http://proofofconceptofwhat.wordpress.c ... ce-photos/

Check it out, it is worth

I had a look around, some interesting info there.

Zonderr strikes back, and has found a way to align faces mouths as well :

http://proofofconceptofwhat.wordpress.c ... alignment/

The result of face alignment (28 faces!) is nothing short of amazing :



PS : Did I say this has been done automatically, of course ? :p