According to Herman Bavinck, there are three ways to speak about the relation of Law and Gospel. The first way is broadly, as depicting the Old and the New Testaments (Herman Bavinck, Reformed Dogmatics, 4:453). The second way is "concretely," which is to say as presented in the text, such that "the law made promises and the gospel utters admonitions and imposes obligations." (Bavinck, 4:454). The third way is in "content," where the two are contrasted as antithetical to each other (Bavinck, 4:453-4). With regards to the third manner, Bavinck writes,
Although they [Law and Gospel] agree in that both have God as author, both speak of one and the same perfect righteousness, and both are addressed to human beings to bring them to eternal life, they nevertheless differ in that the law proceeds from God's holiness, the gospel from God's grace; the law is known from nature, the gospel only from special revelation; the law demands perfect righteousness, but the gospel grants it; the law leads people to eternal life by works, and the gospel produces good works from he riches of the eternal life granted in faith; the law presently condemns people, and the gospel acquits them; the law addresses itself to all people, and the gospel only to those who live within its hearing; and so forth. (Bavinck, 4:453)
We see here that Bavinck agrees fully with the Law-Gospel distinction as being something that comes from the Reformation, which "restored the peculiar character of the Christian religion as a religion of grace" (Bavinck, 4:453).
Bavinck's second way of speaking of "Law" and "Gospel" is concretely, which is to say that the elements of Law and Gospel are so intervowen in the text of Scripture that "the law made promises and the gospel utters admonitions and imposes obligations" (Bavinck, 4:454). We note here that Bavinck is not by this denying the Law-Gospel distinction he affirmed in the previous page, but rather he is acknowledging (1) that the text of Scripture is not always delineated into neat "Law" and "Gospel" sections, and (2) that issues such as "duty-faith" are tough issues to categorize nearly as either "law" or "gospel." Regarding the issue of "duty-faith," the idea that unbelievers have a duty to believe in Jesus Christ and are thus called to do so, Bavinck states that even among the Reformed there is disagreement about how to categorize it. This "preaching of faith and repentance, which seemed after all to be a condition and a demand" was argued to "really [belong] to the gospel and should not rather (with Flacius, Gerhard, Quenstedt, Voetius, Witsius, Cocceius, de Moor, and others) be counted as law (Bainck, 4:454). So some categorize it as "Gospel," which would seem to have the Gospel making demands, while others like Witsius, who I think are more consistent, would categorize it as "Law". The difficulty is due to the fact that Law and Gospel are tightly intervowen in the text of Scripture, such that, while in concept and content they can be distinguished, and SHOULD be distinguished, they often are found together in the text of Scripture and applied together in practical life.
So we see three main senses in speaking about Law and Gospel. Broadly, they refer to the Old and New Testaments. Concretely, they refer to the textual and concrete applications of "Law" and "Gospel" activities. In content, they refer to antithetical realities of "Indicatives" and "Imperatives." For a full orbed view of both justification and sanctification, we should understand these three senses, so that we neither confuse the "Law" with the "Gospel," nor do we flatten out the Christian life as if everything is about just focusing on the complete salvation in the Gospel.