The basic procedure is first to install the Vjoy driver, this sets up the virtual joystick as you probably know. Joystick Gremlin then uses that joystick as 'its' virtual joystick.
so you then install and open JG, and map the X axis to the virtual X axis,Y to Y, Z twist to Z twist, then you add a curve profile to each of those mapped axes. This is all relatively straightforward. The complication comes in when you want to actually bind them in game. You now effectively have two joysticks activating when you move the physical stick, the driver of the joystick and the Vjoy device. This plays all hell with key binding detection when mapping, as you can imagine.
So this is the part that's really hard to explain, but I'll give it a shot...
You turn on a feature in JG called the Command Repeater (rather than do it as a first step as suggested in the vid, I'd do this as a last step after setting up the curves).
What this thing does is repeat an action you do on the physical joystick, using ONLY the virtual joystick. So the computer sees this happening...you move physiucal joystick, it sees both joysticks moving, waits for both to stop then REPEATS the command on the virtual joystick only for a couple of seconds.
So, when it comes to doing the binding in the game, you move the axis you want to map, and THEN you press the button to map it and let the repeater do the axis movement that the game will detect and map. After that, the game ignores your physical joystick driver, as it's not mapped to anything, and the virtual driver does the actual moving, which is as you have set up in JG with curves.
Good luck, shout if you need more help.