Why does my ADS simulation provide no gain?
By placing an ideal DC source at the gate of the FET, you're shorting the AC signal directly to ground. To answer your question in the comments (why is it shorting out the gate?), think about the gate of the FET as an equivalent impedance in parallel with your ideal DC source. The equivalent impedance of the ideal DC source is \$0\Omega\$ (think of Thévenin's theorem), whereas the equivalent impedance of the gate of the FET is considerably larger. As such, you've got a very low-impedance path to ground, so after passing through the feed capacitor, will mostly travel through the DC source to ground.

To avoid this, you should use an inductor in series with your DC source to isolate it from your AC signal path. At higher frequencies, the inductor effectively looks like an open circuit to your AC signal.

In general, it is very typical to see DC biasing fed through inductors on either side of an active device, with capacitors used between stages (of, say, a multi-stage amplifier) such that each stage can be biased independently.