[quoting William Kelly's Commentary on Luke 21; source: BibleHub]
"But, this is not the only point of interest in this appendix to the prophecy. For the Lord has given us the positive proof, by the way in which verse 32 stands here, that "this generation" cannot mean a mere chronological space of thirty or even one hundred years, for it is brought in after the running out of Gentile times and the coming of the Son of man with power and glory, events still unfulfilled. Its force is moral; not exactly the nation of Israel but that Christ-rejecting race which then refused their Messiah as they do still. This will go on till all these solemn threats of judgment are accomplished. It is profitable to remark that here, not in doctrine or in practice only, but in these unfoldings of the future, the Lord pledges the impossibility of failing in His words. The Lord does not say that this generation "shall not pass away till the temple is destroyed or the city taken," but till all be fulfilled. Now, He had [JUST (in v.24)] introduced the subsequent treading down of Jerusalem to the end of Israel's trials at His appearing, and He declares that this generation shall not pass away till then; as indeed it is only then grace will form a new [future] generation, the generation [yet] to come. The more we hold fast the continuity of the stream of the prophecy, as distinguished from the crisis in Matthew and Mark, the greater will be seen to be the importance of this remark."
--William Kelly, Luke 21 Commentary
[end quoting; bold, underline, and brackets mine]
Hope this helps clarify the passage to you.
[note: the phrase "the TIMES of the Gentiles" refers to that which started in 606/605bc, and represents "Gentile domination over Israel"; think" Neb's "dream/statue/image" with Neb as "head of gold"; which will conclude at the END of the future 7-yr trib, at the time of Christ's Second Coming to the earth, in Rev19 (see Rev11:2); distinct from the phrase "the FULNESS of the Gentiles"]