“G2” can mean different things, but the only clearly defined “end” I can state with confidence is for the G2 phase of the cell cycle in biology. For the cell cycle, the G2 phase ends when the cell enters mitosis, at the start of prophase , once a sufficient level of active cyclin B1/CDK1 (also called Maturation Promoting Factor, MPF) is reached and the G2 checkpoint is passed.

If you meant something else by “g2” (like G2 Esports, a specific season, an exam grade, etc.), tell me which “G2” you’re asking about and I can narrow it down.