Series MOSFET switch
You are confused about how a P-Channel mosfet works.

In that configuration, to turn it on you need to apply a voltage less than your supply voltage (28V) but not more than 20V less. By spec, in this application 23V on the gate gets the quoted R values. To turn it off, the gate needs to go back up near the 28V level.

Since you are feeding it 0-5V absolute, that is really -28V and -23V Vgs. So a) it's always on in the simulator, and b) it's dead or dying on a breadboard.

The spikes you see is simply capacitive coupling from the gate.