Its very simple try following code...
sqlconnection con = new sqlconnection(_connString);
sqlcommand cmd = new sqlCommand("Select * from TableName where ["+DropDownList1.SelectedItem.Text+"] = '" + TxtSearch.Text + "'",con);
sqldataadapter da = new sqldataadapter(cmd);
dataset ds = new dataset();
da.fill(ds);
GVDisplay.Datasource = ds;
GVDisplay.DataBind();
I have just changed only one line of your code
sqlcommand cmd = new sqlCommand("Select * from TableName where [" + DropDownList1.SelectedItem.Text + "] = '" + TxtSearch.Text + "'",con);
Hope it works for you...