On Embassy Duty, as the Company "F" OpsO, I had a State Department General Services Officer (sort of the embassy S-4) tell me he was the equivalent of a Brigadier General and demand that I stand at attention when I addressed him and render a hand salute upon arrival or departure of his presence. The Ambassador (a Former Marine Artillery Officer) and I laughed so hard we couldn't breathe. When the Ambassador could finally speak, he said "Not even in pay grade... dumbass." Then we laughed some more.
From my operational background (not legal, not a JAG), the legal requirement to obey legal orders given by superior commissioned officers is well known and established. A GS civilian is not a superior commissioned officer, and is not in the direct chain of command. The only situation where I could imagine a GS civilian having any authority to direct even one push-up would be a doctor or nurse during an examination following an injury to determine readiness, or during physical therapy, but even then these would be considered medical advice and not orders. If you failed to follow medical advice and as a result you harmed yourself or prevented recovery, then your commander could legally punish you, but that's about as close as I can imagine.
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