The 7 years is not the entire tribulation. It's the 7 years because at the start, the AC makes a peace treaty with Israel, which he breaks 3-1/2 years later at the middle of that treaty.
That;'s old news. Christ said the trib's length was going to be shorter. It's not 7 years anymore.
So as you can see, it doesn't talk specifically about the GT at all. I don't know how you came up with the 6th trumpet being the beginning of the GT.
Here is the ending of the 6th trumpet in a different part of scripture:
Rev 11:1 And there was given me a reed like unto a rod: and the angel stood, saying, Rise, and measure the temple of God, and the altar, and them that worship therein.
Rev 11:2 But the court which is without the temple leave out, and measure it not; for it is given unto the Gentiles: and the holy city shall they tread under foot forty and two months.
Rev 11:3 And I will give power unto my two witnesses, and they shall prophesy a thousand two hundred and threescore days, clothed in sackcloth.
Rev 11:4 These are the two olive trees, and the two candlesticks standing before the God of the earth.
Rev 11:5 And if any man will hurt them, fire proceedeth out of their mouth, and devoureth their enemies: and if any man will hurt them, he must in this manner be killed.
Rev 11:6 These have power to shut heaven, that it rain not in the days of their prophecy: and have power over waters to turn them to blood, and to smite the earth with all plagues, as often as they will.
Rev 11:7 And when they shall have finished their testimony, the beast that ascendeth out of the bottomless pit shall make war against them, and shall overcome them, and kill them.
Rev 11:8 And their dead bodies shall lie in the street of the great city, which spiritually is called Sodom and Egypt, where also our Lord was crucified.
Rev 11:9 And they of the people and kindreds and tongues and nations shall see their dead bodies three days and an half, and shall not suffer their dead bodies to be put in graves.
Rev 11:10 And they that dwell upon the earth shall rejoice over them, and make merry, and shall send gifts one to another; because these two prophets tormented them that dwelt on the earth.
Rev 11:11 And after three days and an half the Spirit of life from God entered into them, and they stood upon their feet; and great fear fell upon them which saw them.
Rev 11:12 And they heard a great voice from heaven saying unto them, Come up hither. And they ascended up to heaven in a cloud; and their enemies beheld them.
Rev 11:13 And the same hour was there a great earthquake, and the tenth part of the city fell, and in the earthquake were slain of men seven thousand: and the remnant were affrighted, and gave glory to the God of heaven.
Rev 11:14 The second woe is past; and, behold, the third woe cometh quickly.
Rev 11:15 And the seventh angel sounded; and there were great voices in heaven, saying, The kingdoms of this world are become the kingdoms of our Lord, and of his Christ; and he shall reign for ever and ever.
Everything at verse 13 and before is the 6th trumpet and is the tribulation where the beast rules and the two witnesses have the 1260 days. That is the same length as the 42 months mentioned in verse two as well as in Rev 13. This makes the 6th trumpet's events 42 months long and is the full length of the tribulation.