Edit the profile for the game, click on the Display tab, set the Output to openglnb and the Fullscreen Resolution to the native for your monitor, if you can find it. If not (tbh, dbgl should be changed to ask randr for the available resolutions instead of presenting you with a list of hard coded resolutions), then set it to some random value (e.g. 1024x768) and press OK. After that you will need to locate the configuration file for the game in
(whereas ~/ means your home folder; in your home folder, press CTRL+H to see hidden folders, or select View -> Show Hidden Files from the menu)
where it should be named after its position in your game list -for example if it is listed second in dbgl, its conf file should be 2.conf- open it and change the line
Also you might want to enable the Aspect Correction option, as it will not stretch the game to fit the resolution of your widescreen monitor, although you might like it that way, stretched.
Hope that helps.