Looks like the DAC shows up in the drop down list, when you play the music choosing the USB dac as the playback option, does your dac indicate that it is receiving the bytes - for example, in RME, I know it is receiving because it shows the sampling frequency of the incoming stream… Does your your DAC give an visual indication like that…
Also, obvious question, I think the Gustard DAC has multiple inputs, did you make sure to choose the USB input 
thx