The GMT FAQ [1] has 2023 for a partially complete telescope (3-4 of 7 mirrors) - only the future will tell if there will even be a point of time where the GMT actually is the largest (optical) telescope in the world.

And for certain applications the four VLT unit telescopes [2] can already be used as one big telescope [3][4]. Even with 4 mirrors in place the GMT would only have slightly more collection area than the four VLT telescopes combined (8.4m primary mirrors versus 8.2m primary mirrors).

[1] http://www.gmto.org/overview/frequently-asked-questions/ [2] https://www.eso.org/sci/facilities/paranal/telescopes/ut.htm... [3] https://www.eso.org/sci/facilities/paranal/telescopes/vlti.h... [4] https://www.eso.org/sci/facilities/develop/instruments/espre...