How unusual is Galatians' approach toward the gospel?

Critics of how Evangelicals use Galatians could raise objections to the effect that Galatians is just one document, it's such a short letter, the approach Paul takes there isn't found much or at all elsewhere, etc. On the more liberal side, Galatians could be dismissed as an occasion when Paul lost his temper, a reflection of a bad aspect of Paul's character that we shouldn't emulate, or whatever. From a more conservative or traditional Christian perspective, it could be argued that we should be agnostic about or not base much on certain aspects of Galatians, since it's such a short letter, we don't know much about the context, the themes in question there aren't found much or at all elsewhere, etc.

But even if something Paul said in Galatians were only found there, it ought to be accepted as part of apostolic teaching, what's taught by scripture, and so on. That point could be conceded, yet the alleged rarity of what Galatians says could be appealed to in order to argue that the themes shouldn't be as prominent in our thinking as they are among many Evangelicals. Though I think Galatians expresses views like the foundational nature of sola fide more forcefully than any other source, that sort of material in Galatians is corroborated elsewhere more than people typically suggest.

Let's start with Paul's other letters. Think of how Romans 1:17 characterizes the gospel as "from faith to faith", then quotes Habakkuk 2:4 about living by faith. And notice that only faith is mentioned. You can't gratuitously assume that he meant to include more than faith, especially when he mentions it a few times without mentioning anything else, as in Romans 1:17. Similarly, Romans 11:6 refers to how the adding of works negates the gracious nature of the gospel. Romans 14:23 refers to faith as so important that anything not of faith is sin. Passages like those in Romans make the most sense as addressing foundational issues, what fundamentally characterizes the gospel, not just matters of secondary importance or less. Or think of how frequently Paul refers to faith without anything accompanying it and excludes works, not only in Romans and Galatians, but also elsewhere (Ephesians 2:8-9, 2 Timothy 1:8-9, Titus 3:5, etc.). That sort of attention to affirming faith and excluding works makes more sense in light of Galatians.

And Paul's letters don't just represent Paul's views. Those letters were widely accepted by the original audiences and others. Peter refers to Paul's letters, surely including Galatians, as scripture (2 Peter 3:15-16). Notice the reference to Galatia in 1 Peter 1:1, which increases the likelihood that Peter was including Galatians in 2 Peter 3.

Or consider Paul's activities in Acts. For example, he was involved in "great dissension and debate" (15:2) over the means by which we receive justification. It wasn't just a matter of dissension and debate, but is even qualified as "great" by Luke. That's reminiscent of Galatians.

The author of Hebrews refers to the foundational nature of faith (11:6).

Or think of the gospels, where Jesus frequently commends faith, calls on his disciples to have more faith, tells people they've obtained forgiveness and peace when they come to faith, etc. The account of the Pharisee and the tax collector in Luke 18:9-14 is highly reminiscent of Paul's soteriology, including in Galatians, and Jesus treats the tax collector's approach to God as a universal necessity, not just one approach among others that could be taken. John's gospel was written with the salvation of the readers in mind (20:31), and it's surely not just a coincidence that his gospel highlights so many of Jesus' comments about being justified through faith (3:15-18, 5:24, 11:25, etc.), a theme 20:31 itself highlights.