All perfect praise be to Allah, The Lord of the Worlds, and may His peace and blessings be upon our Prophet Muhammad and upon all of his family and companions.
Gussalah is the water that comes out from impure clothes during washing, and its ruling is as follows:
If the water that comes from impure clothes during washing underwent change in terms of color, taste or smell then it is impure, as agreed by the majority of the Muslim scholars. Therefore, these clothes should be washed and the aforementioned water should be avoided due to its impurity.
However, if that water hasn't changed in terms of color, taste or smell then it takes the ruling of the clothes: If they are pure then it's pure and vice versa. Kindly refer to [Al-Majmooa' 1/159] for Al-Nawaee. And Allah knows best.