First, you may read this very interesting article : http://www.jacaranda-music.com/Schubert.html
The guitare d'amour, also known as arpeggione, was a short-lived instrument with virtually no repertoire written for it, except the famous Schubert sonata. Why ? Well, to know why, the best is yet to try one modern copy : I did it, and once bought an arpeggione. It was so difficult to play that I eventually gave up, and I think that's the reason why this instrument didn't attract composers neither guitarists alike. Some cello players may have less difficulty but I found the bow technique quite impossible to master as a guitarist.
If you're curious, some chineese luthiers sell modern arpeggiones on the web, not too expensive.
Hope this will help.