Boy, there is a lot of confusion on this topic. The first step to understanding prophecy is: Forget all that you have read, seen or heard....EXCEPT THE BIBLE. There is no other touchstone for truth. Having said that...
About "the great tribulation" - according to Revelation those who "came out of" it (7:14) are "a great multitude which no man could number of all nations, kindreds, people and tongues" (7:14). No man could number? Does this sound like a 3 1/2 year period to you? We can number the amount of people on the globe right now! However, over the millenia, there is no way to number the saved.
Revelations goes on to say that "they shall hunger no more, neither thirst any more, neither shall the sun light on them, nor any heat. For the Lamb which is in the midst of the throne shall feed them, and shall lead them unto living fountains of waters; and God shall wipe away all tears from their eyes." (7:16-17) An obvious picture of Heaven after this world has passed away. Read Revelations 21:1-4 for the same theme and the picture of "new Jerusalem." So, if you're not part of the "great tribulation" you will not go to heaven? ABSOLUTELY. Read carefully: "These (the multitude that no one could number) are they which came out of great tribulation AND have washed their robes and made them white in the blood of the Lamb." Are you in tribulation? Have you washed yourself by Christ's blood? Matthew 24, Luke 21 and Mark 13 show plenty of ways to undego tribulation. All of them have been present since Jesus was recognized as Christ. The Bible is stating that tribulation will be underway as soon as Christ ascended and Satan descended - Rev 12:12 "Woe to the inhabitants of the earth! for the devil is come down to you having great wrath, because he knoweth he hath but a short time."
Rapture? First, there is no word "rapture" in the Bible. Second, the idea that it is secret is not what the Bible teaches. Mt. 24:30 "And then shall appear the sign of the Son of man in heaven and then shall all of the tribes of the earth mourn, and they shall see the Son of man coming in the clouds of heaven with power and great glory." Again, Mark 13:26 "And then shall they see the Son of man coming in the clouds with great glory". Again, Luke 21:27 "And then shall they see the Son of man coming in a cloud with power and great glory." Acts 1:11 "Ye men of Galilee, why stand ye gazing up in heaven? This same Jesus, which is taken up from you into heaven, shall so come in like manner as ye have seen him go into heaven." 2 Peter 3:10 "But the day of the Lord will come as a thief in the night; in the which the heavens shall pass away with a great noise and the elements shall melt with fervent heat, that the earth also and the works that are therein shall be burned up." Where is the "secret rapture"? Fact is, it is nowhere found that Christ will secretly rapture anyone, nor that he will take believers from some "great tribulation". Why should He? This is totally out of character. Was Paul spared? Peter? Stephen? John? John the Baptist? The billions of people since the beginning of the world who have kept God and Christ's words? Jesus Himself? No. None were spared death, persecution or tribulation. Read Acts 14:22, Romans 5:3, Romans 12:12, 2 Cor. 1:4, 2 Cor. 7:4 about "tribulation". Fact is, tribulation is glorious and edifying. How can you tell what kind of a person you are without going through trial? How can you tell if you're a Christian without it? Read the final beatitudes (Mt. 5:10-11) on persecutions.