erikl - you are right, one needs to have a rule file corresponding to this GDS file and to this technology.
The most important things that rule file will provide are:
1. connectivity - which metal layer is connected to which other metal layer by which via layer
2. device recognition - so that the software understands where the devices are formed, and what these devices are, and what their layout parameters are.
I have never heard of such a need, to reconstruct schematic from the layout - in normal design flow, a schematic is done and is available prior to doing layout, extraction, etc.
But, in a hindsight, I do know about situations when people need this and they are doing this, on a routine basis - when they are doing reverse engineering. Companies like chipworks.com, performing reverse engineering services, are taking chips, and deprocess them, layer by layer, to create layout of all the layers. Then, they reconstruct the circuit schematic. For that, I guess they need to write rule deck describing connectivity and device recognition.
Also, they do all sorts of measurements - electrical (like I-Vs on individual devices), structural - layer thicknesses, etc., and so on. Then, they package this info into "reports" (structural and electrical reports) and sell it in the open market. Surprisingly, this activity is absolutely legal. Semiconductor companies are using Chipwork services, or do their own reverse engineering on competitors chips routinely, this is a standard engineering practice...
Other than for reverse engineering or similar activities - I can't think of any other applications of schematic reconstruction from the layout...