Hey Jason, that's awesome that you are new to the hobby, and I can assure you there is a lot to learn. Just be patient and you will figure it all out as time goes on.
Onto the issue at hand. Lets start with the binding issue.
Since you have a 6 channel Transmitter (TX) you MUST connect up a standard Rx to the 3GX. The video below will explain connecting up the Receiver (RX) to the 3GX in slight detail. If you need more, just ask and I can try to find my wires and connect one up for you. You have to run a receiver because when you only use satellites, your tail gyro channel is on channel 7, or Aux2 which your DX6 doesn't have. Running a standard Rx allows the tail gyro to be on channel 5, where all others are located. And yes, you have to be able to adjust tail gain otherwise your helicopter will drift all over the place.
As for where to get that dongle, I have no idea who sells this one. You can try looking them up on eBay to see if there are any there. I do have more than one, but I have absolutely no idea where the others are. And I need the one that I have to program my one helicopter with a 3GX on it (maiden last weekend. Because I needed a 3GX helicopter to help others with tech issues lol. The rest of my helicopters are either running the AR7200BX or the most popular in my fleet, the Ikon or Ikon2).