The word seems most liable to be broken down in two ways, and certainly, both may have been relevant.
At the same time, in this context, Gazal, to rob or plunder may have been intended ironically in this context where the use of these stones is specifically described as being intended to expose the secret combinations.
Without knowing the meaning, the intended application of Gazelem is ambiguous in the context where we find it. It could refer to either the seer or the Urim and Thummim, but the above breakdowns seem to suggest a reference to the latter.