First, it treats them as different administration of the one covenant of grace:

3. Man, by his fall, having made himself incapable of life by that covenant, the Lord was pleased to make a second, commonly called the covenant of grace; wherein he freely offereth unto sinners life and salvation by Jesus Christ; requiring of them faith in him, that they may be saved, and promising to give unto all those that are ordained unto eternal life his Holy Spirit, to make them willing, and able to believe.

5. This covenant was differently administered in the time of the law, and in the time of the gospel: under the law, it was administered by promises, prophecies, sacrifices, circumcision, the paschal lamb, and other types and ordinances delivered to the people of the Jews, all foresignifying Christ to come; which were, for that time, sufficient and efficacious, through the operation of the Spirit, to instruct and build up the elect in faith in the promised Messiah, by whom they had full remission of sins, and eternal salvation; and is called the old testament.

6. Under the gospel, when Christ, the substance, was exhibited, the ordinances in which this covenant is dispensed are the preaching of the Word, and the administration of the sacraments of baptism and the Lord’s Supper: which, though fewer in number, and administered with more simplicity, and less outward glory, yet, in them, it is held forth in more fullness, evidence and spiritual efficacy, to all nations, both Jews and Gentiles; and is called the new testament. There are not therefore two covenants of grace, differing in substance, but one and the same, under various dispensations.

See here for more on this distinction.

Another way the Westminster Confession treats law and Gospel is to treat law as all moral commands or duties which human being ought to follow.  In this case there is a contrast between considering the law as demanding perfect obedience and seeing the law as part of the Gospel:

6. Although true believers be not under the law, as a covenant of works, to be thereby justified, or condemned; yet is it of great use to them, as well as to others; in that, as a rule of life informing them of the will of God, and their duty, it directs and binds them to walk accordingly; discovering also the sinful pollutions of their nature, hearts, and lives; so as, examining themselves thereby, they may come to further conviction of, humiliation for, and hatred against sin, together with a clearer sight of the need they have of Christ, and the perfection of his obedience. It is likewise of use to the regenerate, to restrain their corruptions, in that it forbids sin: and the threatenings of it serve to show what even their sins deserve; and what afflictions, in this life, they may expect for them, although freed from the curse thereof threatened in the law. The promises of it, in like manner, show them God’s approbation of obedience, and what blessings they may expect upon the performance thereof: although not as due to them by the law as a covenant of works. So as, a man’s doing good, and refraining from evil, because the law encourageth to the one, and deterreth from the other, is no evidence of his being under the law; and, not under grace.

7. Neither are the forementioned uses of the law contrary to the grace of the gospel, but do sweetly comply with it; the Spirit of Christ subduing and enabling the will of man to do that freely, and cheerfully, which the will of God, revealed in the law, requireth to be done.