I have just been going through some old family papers and have found letters from Henry Gregory, regimental number 1169. They are dated January 1855 from Balta Plain, March 1855 and April 1856 from Haidai Pasha Barracks. They seem to be to his sister Emma.
I'm sure, because of the regimental number, that this could be your great great grandfather.
I'm sorry it is so long after you posted your e-mail, but I have only just found these letters and have started doing my own research. I feel he could also be my great great grandfather.
Perhaps as a further check, please let me know where you believe Henry Gregory was born.
If we can establish that this is your great great grandfather, I will have no hesitation in e-mailing you copies of the letters.
I also have, from the public records office, a memorandum listing his extracts from his regular soldiers' documents.