In general, printing will work (uniflow) seamlessly from an 'RGU Virtual Desktop'. However, if you attempt to print to 'RGU Print' from a published app, it will fail - you'll receive no indication that anything has happened and you'll see no document awaiting to print in your print queue.
The fix is to select to print, not to 'RGU Print', but to 'Citrix UNIVERSAL Printer' - as indicated below:
Having done this, a new pop-up will appear which you then need to choose the print icon on top left of window, this will then offer you the option to print to 'RGU Print' (uniflow):