Matthew 8:29 New International Version
“What do you want with us, Son of God?” they shouted. “Have you come here to torture us before the appointed time?”
Was there a time set to torment the demons of the Gentiles?
Matt 8:29 is one of a short series of verses about the final destiny of the Devil and his angels. The information about this topic is scant and so we should not be too dogmatic.
Thus, there is an appointed time for the Devil (and his angels) to be punished; but for now, Satan and his angels are confined ("imprisoned") to this earth, Rev 20:1-4, 12:7-9.
Further, I see nowhere in Scripture that makes any distinction between demons of gentiles vs demons of anyone else.
APPENDIX - Spirits in Prison
The somewhat enigmatic passage in 1 Peter 3:19 has caused much division among commentators. Here is a very literal translation of V18-20:
Because Christ suffered once for sins, the righteous for the unrighteous, so that He might bring you to God, having been put to death indeed in flesh, but having been made alive by (the) Spirit, in which also having gone, He preached to the spirits in prison, at one time having disobeyed, when the longsuffering of God was waiting in the days of Noah, of the ark being prepared, in which a few--that is, eight souls--were saved through water,
The central question here is, Who are the spirits in prison? This passage is the only verse upon which the doctrine of purgatory rests. So, several further questions are important as well: When did Christ preach – While in the grave or after His resurrection? Is punishment and purgatory implied?
As Albert Barnes notes in his comments, purgatory and hell are not suggested nor implied in this passage; and why select this specific class of people? Further, what use is such preaching to condemned people who cannot change their situation? The time of Christ’s preaching is also quite unambiguous – it was AFTER Jesus’; resurrection. The contrast in the text is clear – Jesus died in the flesh but was made alive in the Spirit.
Who are the spirits? There have been two broad suggestions:
Thus, the interpretation that is consistent with NT terminology is simple: The fact that Jesus rose from the dead was testament and vindication of Christ even to the fallen angels/demons (James 2:19) now “in prison”, ie, confined to earth awaiting their final judgement. They sinned long ago.