The typical starting point for when the “gap” theory between the 69th and 70th week was introduced sources to John Darby, a founder of the Plymouth Brethren. He wrote in the 1830’s. Aside from the historic view, there were others near that time, but still before him, who disagreed with Darby; they were men like Matthew Henry (c1700), John Calvin (c1550), and Sir Isaac Newton (c1700), and bibles like the Geneva Bible. They maintained the “fulfilled historic” view, rather than the “future view”.
From Matthew Henry, Commentary on the Whole Bible, Daniel Chapter IX, section III, to wit,
Cyrus I. Scofield picked up the new teaching and published it. Since the Scofield Reference Bible of 1917, this theory has gained in popularity. Another person who picked up the new tradition was Sir Robert Anderson who published The Coming Prince.
From Matthew Henry, Commentary on the Whole Bible, Daniel Chapter IX, section III, to wit,
And then there is this from John Calvin."1. The times [Daniel's 70 weeks] here determined are somewhat hard to be understood. In general, it is seventy weeks, that is, seventy times seven years, which makes just 490 years. The great affairs that are yet to come concerning the people of Israel, and the city of Jerusalem, will lie within the compass of these years. ... It does serve still to refute and silence the expectations of unbelievers, who will not own that Jesus is he who should come, but still look for another. This prediction [Daniel's 70 weeks] should silence them, and will condemn them; for, reckon these seventy weeks from which of the commandments to build Jerusalem we please, it is certain that they have expired above 1500 years ago; so that the Jews are for ever without excuse, who will not own that the Messiah has come when they have gone so far beyond their utmost reckoning for his coming. " http://www.ccel.org/ccel/henry/mhc4.Dan.x.html
Sir Isaac Newton also believed the 490 years were fulfilled.After the grace of Christ had been obstinately rejected, then the extension of abominations followed; that is, God overwhelmed the temple in desecration, and caused its sanctity and glory to pass utterly away. Although this vengeance did not take place immediately after the close of the last week, yet God sufficiently avenged their impious contempt of his gospel, and besides this, he shews how he had no longer need of any visible temple, as he had now dedicated the whole world to himself from east to west. http://www.ccel.org/ccel/calvin/calcom25.iv.xxxviii.html
The key verse that was changed to allow the change from “fulfilled” to “future” was to whom the “he” referenced. Again, prior to Darby, no one thought the “he” referenced anyone but Messiah.For by joining the accomplishment of the vision with the expiation of sins, the 490 years are ended with the death of Christ. https://www.blueletterbible.org/Comm/newton_isaac/prophecies/daniel10.cfm
Prior to Darby, all of the writers referred the “he” to Messiah. But Darby believed it referred to the prince who destroyed the city in 70 CE or Titus or more generally the Romans. Thus he looked for some sort of revived Roman Empire consisting of 10 states (toes).And he shall confirm the covenant with many for one week: and in the midst of the week he shall cause the sacrifice and the oblation to cease, and for the overspreading of abominations he shall make it desolate, even until the consummation, and that determined shall be poured upon the desolate. Daniel 9:27 (bold mine)
Cyrus I. Scofield picked up the new teaching and published it. Since the Scofield Reference Bible of 1917, this theory has gained in popularity. Another person who picked up the new tradition was Sir Robert Anderson who published The Coming Prince.