Our policy on gain control is not to deceive our customers by providing a software gain control like other apps do. As acknowledged by several posts on the andraudio mailing list, there is no Android API that allows a developer to set the gain, how unfortunate that may be. Some people say that app 'xyz' has gain control, but we have tested by putting our Android devices near a loud audio source and decreasing the so-called gain parameter in these apps until they were far under the red/0dB. The results were that the samples were indeed lower in volume, but you could hear the ADC saturize and the resulting sample was horribly distorted.
Now, we could either implement a software gain where only increasing gain has a use, but we feel that in this way you could also easily distort your signal. The better solution for making audio louder is to use our offline 'Normalize' effect that will prevent any unnecessary distortion that could result from too much software gain during recording.
To switch off auto-gain, you can try using the Voice Recognition audio input selection from the preferences, which may turn off auto-gain and other 'effects' depending on your Android version and hardware manufacturer.
When using a class compliant USB audio interface, the gain can almost always be controlled on your audio device or using the hardware mixer screen. Please read here on our USB audio support!