To be in Go Mode means that the player has collected a set of items and/or characters that will allow them to complete the game. Jets of Time has three different Go Modes, all with different requirements:
Collect the Gate Key, the Dreamstone, and the Ruby Knife. Enter the Tyrano Lair in 65,000,000 BC and defeat Black Tyrano, then proceed to Zeal Palace in 12,000 BC, enter the Ocean Palace, and defeat Lavos. If you are defeated by Lavos, you may watch the cutscenes and then use the bucket at the End of Time to re-engage Lavos or crash the Epoch into its shell.
Collect the Pendant, the Chrono Trigger, and the Clone. Enter Keeper's Dome in 2300 AD and speak to the Nu in the back room who will release Poyozo Dolls to Death Peak. Now, you may enter Death Peak. Complete Death Peak and watch a cutscene during which the Black Omen will rise. Enter the Black Omen from 12,000 BC, 600 AD, or 1000 AD. Complete it and defeat the Mammon Machine and Queen Zeal to roll the credits. In standard flagsets (with the
z flag), Queen Zeal will be the final boss and you will not need to face Lavos after defeating her.
pflag, the Pendant will already be charged and there is no need to take it to the Mammon Machine first)
Collect the Broken Sword and Broken Hilt and recruit Frog into your party. Take the two sword pieces to Melchior in 1000 AD and he will repair the Masamune. Bring Frog to the Magic Cave in 600 AD and watch the cutscene, after which you will be outside Magus's castle. Completing Magus's castle will grant access to 12,000 BC and open the door to Queen Zeal's throne room. Enter the Ocean Palace, and defeat Lavos. If you are defeated by Lavos, you may watch the cutscenes and then use the bucket at the End of Time to re-engage Lavos or crash the Epoch into its shell.