There is actually quite a lot of justification for it. And you are correct, Daniel did not say it. God through Gabriel said it. It takes a proper view of the entire chapter to understand this. But verse 23 is where it's stated specifically.
[Dan 9:23 LSB] 23 "At the beginning of your supplications the word was issued, so I have come to tell [you], for you are highly esteemed; so understand the message and gain understanding in what has appeared.
Gabriel is not being verbose here. There two different things he is going to teach Daniel. First, the message. The words. Daniel is going to gain understanding of what this whole prophecy means. And in addition, he is going to understand the vision or the pattern being described. The word in Hebrew is "mare" and it means something seen, a pattern, a blueprint. The word is used when Moses received the blueprint from God for the menora when the tabernacle was being built (Numbers 8:4)
Gabriel shows Daniel the pattern of the different periods of weeks and the gaps that occur between. See my logo for the pattern which Gabriel gave Daniel. No one else for millennia has recognized this.
That seems to be true in your eyes.
It is man who has determined how the prophecy should be understood and they hang onto the proposition that all of Daniel 9:24-27 is just simply one prophetic word from God delivered by the Angel Gabriel to Daniel.
It is my view that there are five distinct and very separate prophecies given by the Angel to Daniel.
By believing that these prophecies provide information about the year of the birth of Christ, is, in my humble opinion, is flawed.
The prophecies do tell us that "after" the 62 weeks of years that the Messiah would be cut off, i.e., crucified, and will have nothing for Himself. The prophecies do not tell us how immediate the prophecy concerning the Messiah being cut off, will occur after the completion of the 62 weeks of years that the Temple and the City are rebuilt in times of trouble.
The Prophecy of Daniel 9:26b has embedded in its words that the desolation and devastation would be until the end of the war in Heaven which ends with God judging the heavenly hosts and casting those heavenly hosts that do not measure up out of heaven down to the earth where they will join with the Gentile Kings of the earth who are to be judged at Armageddon to be locked up for many days in a pit.
The Book of Revelation in Chapter 20, verses 1-3, tells us that the pit will be Bottomless and that this pit will be locked for a period of 1,000 years.
Paul tells us that all of Israel will be saved after the completion of the 2,300 years of the Daniel 8 prophecy of the Little Horn being given armies to trample both God's Sanctuary and His Earthly Hosts, i.e., Israel, and Israel cannot be gathered once more by God until the completion of the fourth age of the visitation of the iniquities of the fathers during the first two ages of Israel's existence upon their Children and the children's children in the third and the fourth Age/Generation.
Christ in Matt 24:31 gives us a clue in the Lesson from the Fig Tree parable as to when the end of the fourth age would be.
Christ then goes on to tell the Disciples that the next Age/Generation would see the signs that he had spoken about in His discourse on the Mount of Olives.
Now since we have seen figuratively the budding of the new buds on the fig tree which represents the nation of Israel in 1948 AD, we can determine the approximate year that God will begin to Gather Israel to Himself and when the judgement of the Gentile Kings of the earth will occur at Armageddon.
There is much that we can glean from the scriptures if we have ears to hear and eyes to see.
Sadly, we often look at the wrong "signs" that will help us to understand the prophetic words of God and so construct a wrong understanding of the End Times.
The Erza Challenge that is being thrown in our faces by the latest Biblical expert on this forum, to justify his theory of understanding, is just one example of this.
The Daniel 9:24-27 will span three and a half ages, from about when it was delivered, before its completion will come to an end.
Our desire to have a Christ centred understanding of the End Time Prophecies leads us into error with resepct to our understanding of God's prophetic words.
Shalom