I would extract the data values into local variables once, and then use the
local variables in the if...else statements.
Looks neater and will give you better performance (because it doesn't have
to retrieve the value for each statement and convert it to an int)

(Note: I've not checked this in VS2005, so I can't guarantee it will compile
and run)

private void fn_setRankingColor(GridViewRow gvr)
{
   Label lbl = gvr.FindControl("lbl_RANKINGTODAY") as Label;
   int rn = Convert.ToInt32(((DataRowView)gvr.DataItem).Row["RN"]));
   int rnp = Convert.ToInt32((((DataRowView)gvr.DataItem).Row["RNP"]));

   if( rn == rnp )
   {
       // Do something
   }
   else if(rn < rnp )
   {
       // Do something
   }
   else
   {
       // Do something
   }
}

HTH

Ged