If you don't currently have power to your USB without the GROM installed then I would be suspect that you have a wiring harness issue, or it is currently unplugged. Which GROM did you order? There a two variants for GX460. The radio unit is different for 2014+ and for the 2010-13 you need the 2010-2012 labeled unit; it works for all 2010-13 even though they don't include that year on the unit. I have a 2013 as well, everything works on mine. You just press go to USB and it populates once the unit initializes. I don't recall having to update firmware prior to install, that can all be done wirelessly once you can initialize it.
Per GROM
manual:
To INITIALIZE:
1. Put Ignition Switch to “ACC” or “ON” position
2. Wait up to 5 Seconds
3. Put Ignition Switch to “OFF” position
4. Wait 1 minute
5. Turn Ignition Switch back to “ACC” or “ON” Position
6. Press the DISC button until you are able to select the VLINE interface. (may take more than 2 presses of the DISC for accessing as first press is for built-in CD/DVD changer in most cases)
For the last step I believe that is not accurate for our year, you press the AUX-USB button, not DISC