There is an entire chapter, chock full O’ quotations from classic Reformed theologians and footnoted references to other primary and secondary sources, in CJPM devoted to this question. There is also a extensive collection of primary source quotations at my WSC site on this very question.
Here are just a few to whet your appetite:
John Calvin. For Paul often means by the term “law” the rule of righteous living by which God requires of us what is his own, giving us no hope of life unless we completely obey him, and adding on the other hand a curse if we deviate even in the slightest degree. This Paul does when he contends that we are pleasing to God through grace and are accounted righteous through his pardon, because nowhere is found that observance of the law for which the reward has been promised. Paul therefore justly makes contraries of the righteousness of the law and of that of the gospel [Romans 3:21 ff.; Galatians 3:10 ff.; etc.] (Institutes, 2.9.4).
Zacharias Ursinus (1534-83). Q.36 What distinguishes law and gospel? A: The law contains a covenant of nature begun by God with men in creation, that is, it is a natural sign to men, and it requires of us perfect obedience toward God. It promises eternal life to those keeping it, and threatens eternal punishment to those not keeping it. In fact, the gospel contains a covenant of grace, that is, one known not at all under nature. This covenant declares to us fulfillment of its righteousness in Christ, which the law requires, and our restoration through Christ’s Spirit. To those who believe in him, it freely promises eternal life for Christ’s sake (Larger Catechism, Q. 36).
Caspar Olevian (1536-87). For this reason the distinction between law and Gospel is retained. The law does not promise freely, but under the condition that you keep it completely. And if someone should transgress it once, the law or legal covenant does not have the promise of the remission of sins. On the other hand, the Gospel promises freely the remission of sins and life, not if we keep the law, but for the sake of the Son of God, through faith (Ad Romanos Notae, 148; Geneva, 1579).
Theodore Beza (1534-1605). We divide this Word into two principal parts or kinds: the one is called the ‘Law,’ the other the ‘Gospel.’ For all the rest can be gathered under the one or other of these two headings…Ignorance of this distinction between Law and Gospel is one of the principal sources of the abuses which corrupted and still corrupt Christianity (The Christian Faith, 1558)
William Perkins 1558-1602). The basic principle in application is to know whether the passage is a statement of the law or of the gospel. For when the Word is preached, the law and the gospel operate differently. The law exposes the disease of sin, and as a side-effect, stimulates and stirs it up. But it provides no remedy for it. However the gospel not only teaches us what is to be done, it also has the power of the Holy Spirit joined to it….A statement of the law indicates the need for a perfect inherent righteousness, of eternal life given through the works of the law, of the sins which are contrary to the law and of the curse that is due them…. By contrast, a statement of the gospel speaks of Christ and his benefits, and of faith being fruitful in good works (The Art of Prophesying, 1592, repr. Banner of Truth Trust,1996, 54-55).
If the Reformed theologian J H Alsted was right to say that justification was the article of the standing or falling of the church, then the law/gospel distinction is the term of the standing or falling of the gospel. Without this distinction there is no good news.
As a matter of history, without this distinction there Reformation would not have happened. The medieval church spoke about the gospel for 1000 years but when they said “gospel,” they did not mean, “the announcement of the unmerited and undeserved salvation of the elect on the basis of the imputed righteousness of Christ received through faith resting and receiving alone.” The medieval church defined “gospel,” as the “new law.” The Bible was said to be composed of old law and new law. The distinction between them is that there was said to be more grace, with which we were said to be infused by the sacraments to enable us to cooperate with grace, in the new covenant to keep the new law. Truly, in the medieval church we were “in by grace” and we “stayed in by faith and works.”
The Reformation rejection of that scheme was foundational. Are there differences between the confessional Reformed understanding of the law/gospel distinction and the confessional Lutheran construction? Yes, but it is categorically not the case that the law/gospel distinction is only Lutheran.