When you let go of the sticks out will not instantly stop. It will self correct and become stationary before holding. The higher your attitude gains in pitch and roll the faster it will level out.
Also if you have not mounted your GPS dead strait you can suffer from 'the toilet bowl effect' where it goes in a small circle. check that its aligned, and do a basic calibration in the assistant.
For your gains set both attitude gains to the x1 channel and use a dial on your tx to sweep then whilst test flying to get it right.
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