Sorry, I haven't come up with a lot. As far as I can tell
from a quick re-look, the phrase Bonhoeffer actually uses is "God as a
stop-gap" in _Letters & Papers ..._ (pp.103-104 in the Fontana edition).
To be precise one would have to check the German, which I don't have at
hand now. _The Westminster Dictionary of Christian Theology_ has a
short article on "God of the Gaps" but no history of the phrase. I
suspect where one needs to look is in literature connected with the
"Death of God" debates of the late 60s.
George L. Murphy