I think the question in the title of the thread makes answering difficult. "Mosaic Covenant: Law or Gospel?" Well, are you defining "law" and "gospel" in the dualistic Lutheran manner that WSC seems to follow? If so, I believe it's a false dichotomy. There is law and gospel in the Mosaic covenant ("I brought you out of the house of Egypt, out of the house of bondage; Thou shalt have no other gods, etc.") just as there is law and gospel in the new covenant ("Neither do I condemn you; go and sin no more").

And if you want to be confessional, take note that the Westminster Standards never define "law" and "gospel" in the dualistic Lutheran sense. And what's more, the Standards are pretty clear that the Mosaic covenant is an administration of the covenant of grace, while saying nothing of it being an "administration" of the covenant of works (in any sense whatsoever). Seems to me that the doctrine of the republication of the covenant of works at Sinai is at best extra-confession and at worst contra-confessional (if the Westminster Standards are your standards).

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Casey Bessette
Westminster OPC • West Suburbs of Chicago • My Blog: Paradise Regained

"It is part of the calling of the ekklesia to learn to know the love of Christ that surpasses all knowledge and also to make known within the world of science 'the manifold wisdom of God' in order that the final end of theology, as of all things, may be that the name of the Lord is glorified. Theology and dogmatics, too, exist for the Lord's sake." — Herman Bavinck, Reformed Dogmatics, vol. 1, p. 46