The listed Accepted File Formats for DKRed do not currently list the Kicad .gbr or Protel filename extensions as accepted file formats.
Please note DKRed is compatible with the standard, default Kicad .gbr filename extensions, the algorithm detects the keywords of the layer filename and assigns them to the correct layer. Kicad works with DKRed!
It is also mostly compatible with the ‘Use Protel filename extensions’ in the case when I tested it out it assigned most of the layers to the correct layer type except the board outline ‘edge cuts’ layer was assigned to Board Mechanical.
The quick easy fix for this is just manually changing this layer to assign it to Outline File.