J.C.,
Your solenoid could indeed be bad, or you could have a bad electrical connection in the plug and play system.
1.) If you have a plug and play gateman the connecting cable coming out of it should look like this, except that the end of it with the bare wires seen here is actually inside your gateman's base (so it will not be visible without opening the gateman up:
You'll notice that the plug has three pins. This means that there are two circuits, one for the light (which is usually not switched, so the light is on any time the track is powered), and one for the solenoid to open the door, etc. (which is switched on when a locomotive or car is on the activator track and straddling the two outer rails, and off when no locomotive or car is present.)
Since your solenoid is apparently not working, but the light is, it could be that this switched circuit is open (disconnected) at some point along the cable. Check to make sure that the connector is seated fully because if it's not one or both of the two circuits could be incomplete, in this case the power circuit to the solenoid but not the circuit to the lamp.
2.) It could also be that the activator track has a disconnected wire inside it on the solenoid circuit. In this case try one of the other outlets (it has 3 of the 3-pin type):
3.) It may also be that the switch setting on the activator track could impact things here. If you have it set to 'AUX' the activator track is expecting you to connect an external power brick to the to it for powering the connected accessories independently. If you have no brick connected then you must put the switch in the 'TRK' position for power to flow to the solenoid (sourced from the main transformer through the track) when a loco or car is present.
At this point if you're asking yourself the question "But the light works; why should the position of the switch matter?" it may be that the light is always powered off the track, but the solenoid can be powered either way. If I remember correctly Lionel's instructions for the activator track are not too clear on this.
Mike