What I understood from your description was

You are using a nested gridview in which you are doing a postback to load some child gridview and once you select the row in the child gridview,

You want to hide the others.

You can do like this

find the Child gridview in the code behind and in the selectedIndexChangedEvent,

find the Gridviewrow selected and then

gv.Rows.OfType<GridViewRow>().ToList().Where(a=>a.RowIndex !=row.RowIndex).ToList().ForEach(b=>b.Visible=false)

it will get all the rows in teh child gridview and hide all rows except the row selected.

Correct me if I understood your requirement wrong.