...tribulation converts...
Note that it doesn't show anyone repenting during the tribulation, but says that the unbelievers "repented not" (Revelation 9:20-21, 16:9-11), and that at some point in the tribulation "God shall send them strong delusion, that they should believe a lie: That they all might be da*mned who believed not the truth, but had pleasure in unrighteousness" (2 Thessalonians 2:11-12). So it's possible the Christians we see in the tribulation are us, saved before the tribulation began. Jesus doesn't promise us a rapture before the tribulation.
[*The posting program won't accept the Biblical word d-a-m-n-e-d.]
...The Times of the Gentiles...
Note that no scripture says that the times of the Gentiles ends before the tribulation. "Jerusalem shall be trodden down of the Gentiles, until the times of the Gentiles be fulfilled" (Luke 21:24). Because Jerusalem shall be trodden down by the Gentiles during the coming tribulation: "it is given unto the Gentiles: and the holy city shall they tread under foot forty and two months" (Revelation 11:2), the times of the Gentiles can't be fulfilled before the tribulation.
...the rapture is a part of the first resurrection...
Note that no verse promises us a resurrection before the 2nd coming.
I believe the Bible precludes that our resurrection be before the 2nd coming because it says the resurrection and changing of all Christians into their immortal bodies will occur at a single point in time, at the "last trump" (1 Corinthians 15:52), at the 2nd coming of Christ (1 Corinthians 15:23), and that "the first resurrection" will also include all of us Christians who will die in the tribulation (Revelation 20:4-5).
I believe the pre-trib doctrine would require that 1 Corinthians 15:23, 52 on the one hand, and Revelation 20:4-5 on the other, be referring to two different bodies of Christians regarding two different comings of Christ, thus making the one body of Christ two bodies (Ephesians 4:4-5), the 2nd coming a 3rd coming (Hebrews 9:28), the last trumpet the 9th from last (1 Corinthians 15:52; Revelation 8:6; Matthew 24:31), and the 1st resurrection the 2nd (Revelation 20:4-6), so that the pre-trib doctrine would not be making sense of scripture.
Note that the Bible doesn't refer to an "age of grace," but instead says "that in the ages to come he might shew the exceeding riches of his grace" (Ephesians 2:7). I believe the covenant of grace (Hebrews 10:29) is the "everlasting covenant" (Hebrews 13:20), and the gospel of grace (Acts 20:24) is the "everlasting gospel" (Revelation 14:6). We Christians who will be in the tribulation who have washed our "robes, and made them white in the blood of the Lamb" (Revelation 7:14) and have "the faith of Jesus" (Revelation 14:12) and are "in the Lord" (Revelation 14:13) will be in his grace.
"Grace, which was given us in Christ Jesus before the world began" (2 Timothy 1:9).
And grace will continue even after the tribulation, for at the 2nd coming Jesus "will pour upon the house of David, and upon the inhabitants of Jerusalem, the spirit of grace: and they shall look upon me whom they have pierced" (Zechariah 12:10).
Note that the Bible doesn't refer to a "church age," but instead says that the church is "throughout all ages, world without end" (Ephesians 3:21). Note that we Christians are referred to throughout Revelation (6:11, 7:14, 9:4, 12:17, 13:7-10, 14:12-13, 15:2, 16:15, 18:4, 20:4), and there are no Christians outside of the church (Ephesians 4:4-5).