RE: GY401 problems with R50 Titan
I'll start at the end of your post. The play that you are describing on the tail shaft needs to be fixed. Good news is that it's easy and common. You put the end of the pulley on backwards, so it's not seating right and giving you that play. Open the tail box and compare the way the pulley looks to how it's shown in the manual, it'll be fairly obvious.
Here's how to set up that gyro. First, put the switch to DS mode. Then move the stick over all the way and adjust the limit setting so that there isn't any binding. Ideally you want it over 100%, if it's not, then move the ball on the servo horn in one spot, and try it again. I'm assuming you have the direction set properly, or you'd have much larger problems.
Go into your radio, and go make sure that the trim is centered on the rudder channel, including sub-trim. Go to the gyro screen, turn it on, and put the gyro switch on your TX up. Set the value on the screen to 70%, flip the switch down, and set it to -70.
Now go fly. Most likely, it'll all work fine, but it may take some fine tuning on the gyro screen. If the tail is wagging side to side quickly, turn the value down, if it isn't holding, turn the value up. Don't run it at 100%, it won't work. 99.9% of the helis I've set up with 401s (probably hundreds of them) will fly fine with the settings I gave you above, and won't need adjustments.