Yes it's possible to send audio out to the G68. It will still include audio over HDMI to the TV.
You select the audio output from the HD621 to the G68.
Right click on the HD621 and select "properties", or select "connect" and walk through the setup.

Mconfig Diragram detailing a HD621

Main: Marantz Cinema 50, Meridian 271, DSP7200.2, DSP5200HC, DSP5000 rears, Meridian 210.
PC/Headphones: Mytek Liberty II, Sansui CA-F1, Sansui B-2301, Sennheiser HD800, Kef R3