I know that after washing head part in gusl we have to stop or put the water in a side before washing right part of the body.

If I am already washing the left part of my body and I doubt if I stop the water after washing my head.
Do I have to repeat the gusl?
Or no because the act already finished?

If you doubt you can wash the right side, then wash the left side again.

If Ghusl is finished, no need to repeat