I‘m going nuts while working on the basically really simpe task of reading ADC voltages on GPIO (AIN3) between 0 and 1.25V.

The voltage should fit well in any range so I thought I could just use a simple GAIN_1 setting with internal reference. But I don‘t get the right results. At least not if readings should give millivolts with no further calculations needed.

please help! Can someone show me the correct settings?

But I don‘t get the right results. At least not if readings should give millivolts with no further calculations needed.

Have you looked at SAADC example? Also, have a look at the SAADC chapter in the product specification. The formula to calculate the output can be found here.