This is outside my area of expertise. Back when video signals were analog, graphics cards contained a DAC (either integrated in the GPU or as an external device in the days before GPUs). Nowadays, video signals are digital, so I do not think there are DACs in modern GPUs.
Independent of the use of DACs in graphics cards a DAC is not simply reversible into an ADC. At best you could construct a ADC mechanism that utilizes a DAC as one of its constituent parts. For example, use a counter, a DAC, and a comparator: feed counter output to DAC, compare resulting voltage from DAC to input signal, stop counter from incrementing when output from DAC exceeds input voltage. The counter value thus represents the input voltage. You would need additional electronics to construct such a feedback loop to achieve ADC functionality, but a DAC integrated into the graphics card does not offer such additional circuitry.