The GS2011M's SAR ADC has two options for the reference voltage, which is also the full scale voltage. The first option is to use the internal bandgap voltage reference, which is 1.4V. The second option is to use he VIN_3V3 voltage as the reference. The first option is good if the accuracy of the vIN_3V3 voltage is not trusted. The second option gives a wider voltage range, and is particularly good with sensors that use the same VIN_3V3 voltage as their reference (e.g. a thermistor).
The GS2100M's sigma-delta ADC's have differential inputs, and full scale is nominally 2V differential. Denoting the differential inputs as P and N, reading 0x7FFF occurs when P = N+2V, and reading 0x8001 occurs when P = N-2V.