The latest release of MarcEdit (version 5.8) includes an RDA Helper:
The RDA Helper is an in development tool that provides the ability to RDA’ize AACR2 records. The function allows users to select from a broad range of RDA field options. RDA Helper includes automatic GMD generation
Terry Reese (the genius behind MarcEdit) has done a video to show you this new feature:
Check out the latest version of MarcEdit for this and other new/improved features.