So far in 2015, GOG has released 97 titles. Of those, 25 (26%) were day 1 releases, 15 (15%) were pre-2K ones and 29 (30%) were at least 10 years old at the date of GOG release.
More info can be found in this thread
, and the data is compiled here
GOG has also signed a few new publishers that do have quite a few classic games, so the classic releases will not stop, even if at times it does seem like they are thinning.