Gray water can usually be safely let out on the ground without any issues.
The ground will naturally filter the gray water and it will not harm the environment nor will the gray water being released onto the ground harm any other water sources.
The earth is a natural filter and before the water reaches the water veins in the earth it is filtered naturally so it cannot contaminate water.
As long as the gray water is not directly dumped into a river or water well then the water will be just fine on the ground.
Some counties and places may make it illegal to dump gray water on the ground which is not right because gray water is not gonna harm the environment.
But in some places you can be fined for letting gray water onto the ground.
Some people divert their shower water to the garden to allow the plants to get water and save other water.