The original GOG release (1.004) had some additional fixes and changes compared to the disk version of DD, but introduced a serious new bug. That was fixed in an update (1.005), but the changes and some of the bug fixes in the disk version patches were lost. In terms of bug fixes the Steam version and current GOG version (1.006) are pretty much the same as the previous GOG version. See
here and the following few posts.
