The short answer is that the Law/Gospel distinction is found in many important Reformed writers. One can certainly argue that the Three Forms of Unity are based on this distinction, in pointing out (in the HC, for instance) misery (which we find out by the law), salvation (pointed out to us by the Gospel), and gratitude (the Law/Gospel distinction does NOT eliminate the third use of the law, contrary to what some might think). Here are some quotations that clearly indicate that the Law/Gospel distinction is Reformed.
First up, Ursinus, one of the two authors of the Heidelberg Catechism (concerning question 2, which lays out the structure of the HC):
This question contains the statement and division of the whole catechism and at the same time accordes with the division of the Scriptures into the Law and Gospel. (Commentary on the Catechism, p. 20)
Hence it is manifest that we must commence with the preaching of the law, after the example of the Prophets and Apostles, that men may thus be cast down from the conceit of their own righteousness, and may obtain a knowledge of themselves, and be led to true repentance. Unless this be done, men will become, through the preaching of grace, more careless and obstinate, and pearls will be cast before swine to be trodden under foot. (Commentary, pg. 21)
Next up, Turretin, in speaking of the covenant of grace:
(In talking of Galatians 4:24) “He disputes against the false apostles who confounded the law and the gospel.” (IET II, p. 236)
There is not the same opposition throughout between the Old and New Testaments as there is between the law and the gospel. The opposition of the law and the gospel (in as far as they are taken properly and strictly for the covenant of works and of grace and are considered in their absolute being) is contrary. They are opposed as the letter killing and the Spirit quickening; as Hagar gendering to bondage and Sarah gendering to freedom, although the law more broadly taken and in its relative being is subordinated to the gospel. (IET II, pp. 236-237).
W.G.T. Shedd (in the third edition), p. 824 quotes approvingly (“the following excellent statement of the law and the gospel as means of grace”) the Lutheran Formula of Concord article 5 (for which, see Schaff, Creeds of Christendom, volume 3, pp. 126-130, which is followed, interestingly enough, by an exposition of the third use of the law, article 6; the Lutherans did NOT deny the third use of the law!).
Johannes Vos, in his commentary on the WLC, pp. 235-236:
13. What is the place of the moral law of God in a scriptural program of evangelism? While the word evangelism means “proclamation of the gospel,” we should realize that the gospel is meaningless without the law. Gospel means “good news”: that is, good news of salvation from sin. Sin is the transgression of the law: without conviction of being transgressors of the law, people will feel no need of the gospel; without knowledge of the moral law of God, people will not feel themselves to be transgressors of the law. Therefore no program of evangelism is sound os scriptural which does not emphasize sin as the transgression of God’s moral law. Much present-day “evangelism” has little to say about God’s law, sin, and repentance; instead, the tendency is to speak only about “accepting Christ.” A return to the old emphasis on God’s law is urgently needed. Without it, there cannot be a genuine revival of the Christian faith.
Notice that Vos equates the Law/Gospel distinction with the first use of the law. This means that the WS do indeed teach the Law/Gospel distinction as the first use of the law. Therefore all six forms of unity teach the Law/Gospel distinction. The reason some are uncomfortable with the Law/Gospel distinction is that they feel it does away with the relevance of the law for the Christian. It does no such thing. As we saw even in the Formula of Concord, the Law/Gospel distinction is immediately followed by a discussion of the third use of the law for the believer. Similarly, the treatment of the Ten Commandments in the HC is found in the section on gratitude, NOT in the section on misery, or salvation. Ursinus obviously felt no schizophrenia for arguing in this fashion (see the quotes above). On to other authors in Part 2, which will follow shortly.