In addition to the answer explaining nothing up my sleeve number of pi million, it's worth to add that the gas limit is not set in stone.
As the yellow paper equations describe the gas limit can adjust with every block plusminus 1/1024 which is around 0.09%.
In case of a very high network utilization, the network could adjust the gas limit by a factor of around 131 per day, assuming there are around 5k blocks per day:
1.0009765625 ^ 5000 = 131.68725950103537531831
See also live, what happens if you increase the gas limit:
