The most succinct way to do this is just with an anonymous class, if you are throwing this to some client side code and not really messing around with this exact object again elsewhere in server side code this is the easiest way to handle it.
var outputString = JsonConvert.SerializeObject(new {
success=1,
error=0,
gestor="test",
cliente = (from System.Data.DataRow i in com.Execute("select * from utilizadores").Rows
select new {
nome=item["first_name"],
apelido= item["last_name"]
}).ToArray()
});