Those without excuse must only refer to the Jews (Jeremiah 11:20) from the “creation of the world”, which is covenantal cant, since the promise was committed to them first (also see John 1:16-18). Also see John 1:11-12, 10:24-27, Romans 11:28-36, Luke 19:9-10, etc. Unto them were committed the oracles of God (Deuteronomy 29, Romans 3:2). Paul summarizes his arguments in a retort in Romans 3.

Notice Romans 1:20 is an obvious allusion to the Old Testament:

Isaiah 64:4 For since the beginning of the world, men have not heard, nor perceived by the ear, neither hath the eye seen, O God, beside thee, what he hath prepared for him that waiteth for him.

The things were invisible until the Gospel was revealed (Isaiah 40:1-21, 45:3, 48:3-8, 53:1-3, 56, 64:4, Jeremiah 33:6, Habakkuk 2:2-4, Matthew 11:25-27, 13:35, Luke 10:21-24, Romans 1:16-17, 1 Corinthians 2:7-9, 2 Corinthians 4:3-7, Galatians 3:23-25, Ephesians 3:2-9, Colossians 1:16, 1 Peter 1:9-12, 1:20). The judgment is within the context of the Gospel’s effectual delivery (1 Kings 18:10, Psalms 19:4-6, 22:27-28, 98, Isaiah 49:6, 52:9-10, Matthew 28:19, Mark 16:20, Acts 26:20, Romans 1:8, 10:18, 15:19, Colossians 1:6, 1:23) and the particular implications for Jews and Gentiles (John 10:16) into a single dispensation (Ephesians 2:13-22, 3:6). The single dispensation is out of a single judgment by the same universal terms (Jeremiah 9:25, Acts 7:30-31, Hebrews 9:27).

The Central Theme
Paul concludes that all are justified by faith alone. To make this argument, he has to make the practical application that the Jews cannot be justified by the Law. The premise is that the righteousness of God is revealed by God in the Gospel of Jesus Christ unto Salvation. If the Jews cannot abide by the Law, then only by faith (and not the works of the Law) can any be justified. If a gentile has faith, and if the Law (exclusive to the Jews) is not by which any are to be saved (but faith alone), then we must conclude God justifies them too (Romans 2:25-29).

The error liberals fall into is the assumption that Paul needs to make a case for why the gentiles are without excuse.
Gentiles are dead already without the Law (Romans 3:12 For as many as have sinned without law will also perish without law…). See Ephesians 2:11-12.
Paul makes his case for this in Romans 5:12-14.

God established His standard of judgment in the Law to the Jews, and the consequence of this is that the whole world and not only the Jews are become guilty before God (Jeremiah 9:25, Romans 3:19, Hebrews 9:26).

The Law alone is the standard (Romans 2:2) by which the whole world be judged (Romans 3:19). Regardless of whether they hear the Law or not, it is only the doers of the Law are justified (Galatians 3:12). Since none are capable of keeping the whole (Deuteronomy 27:26, James 2:10) Law, Scripture concludes that none are justified by the works of the Law (Galatians 2:16, 3:10-11). Yet liberals have a trend to delegitimize the standard of the Law by excusing those who do not hear the Law. Nevertheless, it is not the hearers of the Law that are justified, but those who have Faith in Jesus Christ (Romans 2:13, Galatians 2:16).

Evidence from Romans 3:19-20

That is, the Jews are judged by the Law given to them, and the gentiles without the Law are still despondently dead without the Law, because all are under sin (Acts 17:26-31, Romans 3:9-20).

“Whom does Paul address in vv. 1:18-32?

Gentiles. It is clear because all their revelation about God comes from nature.”

I disagree.

The people from Romans 1:18-36, for example, refer specifically to the Jews alone, and not the Gentiles.

First, in Romans chapter 1:18-2:25, Paul explains why the Jews were without excuse. They alone had what may be known of God manifest to them, and God only showed the Jews through His Law (Romans 3:1-2). Not the gentiles (Psalms 147:19-20). He compares those who had the Law, but suppressed it in unrighteousness and did not glorify Him or thank Him because there foolish hearts were darkened. This portrait of the Jews in Romans 1 alludes to Old Testament depictions of the covenant breaking people (Daniel 5:23, Jeremiah 3:17, 7:24, 9:14, 11:8, 13:10, 16:12, 18:12, Jeremiah 4:22, 5:4, 5:21, 10:8, Lamentations 2:14, Isaiah 5:20-21, Jeremiah 8:8-9, Ezekiel 8:10, Psalms 106:12-40, Isaiah 2:8, Ezekiel 16, Jeremiah 6:19, 6:28-30, 31:32, etc.).

Natural theology- Some posit that God reveals Himself to all of humanity universally (see Matthew 11:25-27, Luke 10:21-24, John 14:17, 2 Thessalonians 2:10-15) by nature (see Matthew 16:16-17, 1 Corinthians 2:12, Galatians 1:16). They often implicate that the Scriptural revelation is unnecessary, inadequate, or insufficient. The routinely deist theology often presumes humans have the free-will propensity to choose their own destinies (see Job 14:5, John 1:13, 6:44-45, 1 Corinthians 1:26-29, Ephesians 1:11, 2:5, 2:8-9). This variety of deism would seem to imply that the Gospel has not been adequately revealed and is rivaled by material creation (see 1 Corinthians 2:12-13). They sloppily induct that God probably offers to some a substitute for the Scriptural Revelation of the Gospel to guide mankind (see Romans 16:25-26).

God revealed Himself first to the Jews of the Old Testament
Psalms 147
19 He declares His word to Jacob,
His statutes and His judgments to Israel.
20 He has not dealt thus with any nation;
And as for His judgments, they have not known them.

Deuteronomy 4, 31:11-13, Nehemiah 9:13-17, Psalms 78:3-7, Isaiah 8:20, Ezekiel 3:4-6, 20:9-28, 36:22-33, Amos 3:1-2, Luke 16:29-31, Romans 2:18, 3:2, 9:4.

-Gentiles were in darkness
Ephesians 2:12 that at that time you were without Christ, being aliens from the commonwealth of Israel and strangers from the covenants of promise, having no hope and without God in the world. Ephesians 5:8 For you were once darkness, but now you are light in the Lord. Walk as children of light

Psalms 10:16, 79:6, 147:19-20, Proverbs 29:18, Isaiah 5:1-7, 45:20, Acts 14:16, 26:17-18, Ephesians 4:17-19, 1 Peter 2:9-10.

Grace to you,
Bible-Touchstone