First, make sure the status light on teh 3GX is green. If it's green, great. If not, go into your gyro menu on the DX6i and set your number ABOVE 50. Start with 65% on position 0. This should give you a green light.
If you followed my video for the DX8, then you have your gyro set to 35% and that is not only RATE mode on the gyro, but it's a VERY HIGH rate mode and will cause this wag.
So go in and check your gyro settings.
Another thing, you MUST be using a separate Rx with teh 3GX. If you are ONLY using satellites, then this is your issue. When you use satellites only, then your gyro channel is #7, and you don't have a 7th channel. Channel 5 is actually now your governor. However, when you use a separate Rx, the gyro channel goes to channel 5 where you need it. Something else to check.
Let me know if this helps.