This ultimate fusion had been executed by Goku and Vegeta as Super Saiyan 100. This fusion is so strong that it can shake omniverse and damage multiverses even before completion of fusion. Its strength is so high that i t only possible to remain fused for 1 nano second. Elder Kai advised Goku and Vegeta to fuse in black hole to cheat time and prevent damage to universe from fusion.
They succeeded and found that their cells in body were used to time in black hole which aged much less (almost 100000 times slow) ,giving time boost for fusion to last for complete 45 seconds. Gogeta returned to earth and defeated Buu who had absorbed Golden Cooler and Broly Super Saiyan 8.