This is a double bluff, not a single bluff.
They first bluff is the false bluff that the gate is not there, and the second is the gate being there.
Details:
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It was originally obvious to the observers (the OOTS) that the gate was in the pyramid. This contains no bluff.
When they reach the gate room, it says the gate is not there. This makes the evidence that the gate was in the pyramid into bluff.
The gate is actually in the pyramid, despite the sign. double bluff.
Simpler:
All evidence says the gate is in the pyramid, but once inside, it appears the entire pyramid is a bluff (#1).
In fact, the sign saying that the pyramid is a bluff is a bluff (#2), and the gate is within the pyramid.