There are two main mistakes here:
1) This is maybe not a mistake but in the simulation your samples per bit is only 16 and the width of the gaussin pulse is 0.04 of a bit period. The sampling period is then 3.125 ps and the gaussian pulse width is 2 ps. The Gaussian pulse is much shorter than the sampling period so the signal right off the bat will not be represented very well. I would increase the sample per bit to much larger like at least 128.
2) If you check the normalize parameter in the ADC component you should see the discretization. The way it calculates the resolution is a little strange to me. Basically, the resolution of the output signal is equal to 2/(2^(n)). So in your case with 3 bits gives 0.25 A.U. Your electrical signal is very large because of the amplifier, so it is hard to make out the discrete nature. Checking the normalize option will bring the max amplitude down to 1 A.U. and it will be much more clear.
Regards