Welcome to the Pentaxforums and greetings from Munich.
Going by the DxO products page your GR is fully supported, including its DNGs. But what version of Optics Pro are you using? Looking into the
software release notes, support for your camera was added with version 9.0.1 (current version is 9.5.2). Optics Pro 8 also supports Ricoh GR from version 8.3.2 onwards. Both with firmware 2.0.3, which seems to be the last version Ricoh did any changes to DNG.
Maybe you just need to update the DxO software? If that doesn't help you could
contact DxO to open up a support case, I guess.
Good luck with your problem and have fun on the boards.