In the book "Topos Theory" of Peter Johnstone (Topos Theory, LMS Monographs no. 10. Academic, 1977) one finds at page 41 in Chapter 2:

"For a detaled account of internal categories from a 2-categorical point of view, the reader is referred to the monumental work of JW Gray [168]"

The work of J.W. Gray has to be very important and significant (in those years Gray was one of the great masters of category theory) as well known and whose output is only the first part in Formal Category Theory: Adjointness for 2-Categories. Series: Lecture Notes in Mathematics, Vol 391.

Does anyone have news of such notes of J.W. Gray? Of course I would like to read these.