Ages ago the PLGO group compiled a list of GCS volumes online. This vanished recently, as I discovered when I went looking for it. A version still exists in ScribD, but, you know, I don’t find stuff in ScribD that accessible.
Rather than go without, I have transcribed the list and placed it on a page on this blog here.
Updates are welcome.
UPDATE: I have just been through Internet Archive using terms such as “eusebius werke” and “origenes werke” and filled in most of the gaps before 1923. Methodius is one of only two gaps (of course it would be one that I want to look at). It seems that the people at IA have been very busy, bless them!