Technically the park closes at dusk, but I've seen many photos of the stars from the top, so they might not close the gate at night. It's a dangerous drive to make at night, though.
When they lock the gate they usually run up to the top to Check for stragglers.
Officially the park closes at dusk so you are not supposed to stay back after it gets dark.
I wouldn't recommend risking the drive to the top in the dark. The road is fairly narrow with steep falloff on one side, and no guard rails or trees to protect from tumbling down in case of a mishap. Also, given the absence of mountains or other obstructions, the surrounding area might be good enough for star-gazing on a clear night.
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