The seven last plagues from the wrath of God is described here in this chapter. It is not a tribulation, it is a wrath and punishment, or in some ways a simple exhibition of Gods power.
Then I saw another sign in heaven, great and marvelous: seven angels having the seven last plagues, for in them the wrath of God is complete. Rev 15:1 |
The next verse confirms the ones who rejected the mark of the beast are safe and have joined the others who are in a cloud hovering somewhere over earth. They stand on a sea of glass just the same as everyone else. After all, we are only human and we need gravity to keep us down. I expect this sea of glass will have some sort of gravitational field to it.
And I saw something like a sea of glass mingled with fire, and those who have the victory over the beast, over his image and over his mark and over the number of his name, standing on the sea of glass, having harps of God. Rev 15:2 |
The rest of the verses in this chapter shows where the plagues will come from and say that no one will be able to enter the temple until all of these plagues are done.
The temple was filled with smoke from the glory of God and from His power, and no one was able to enter the temple till the seven plagues of the seven angels were completed. Rev 15:8 |
Pretty much, the jest of this chapter is to show the timing of Jesus in relation to the time of the mark of the beast and to show that the wrath of God is about to be completed. Again, the wrath of God is not the tribulation. It's a punishment meant for Satan and his children and followers. At this stage all the believers are gone and waiting for Jesus to finish so they can return to earth to live with him for the next thousand years.