Get Optionset Label From Another Entity Through Lookup

Hi guys, i created 2 entity. On the child entity i place look up value that will look data from parent entity. Now i tried to retrieve optionset from parent entity and place it into child entity. 

Here is the code i have tried :

postMessageImage.Attributes["new_gender"] = customer.GetAttributeValue<OptionSetValue>("new_gender").Value.ToString();
postMessageImage.Attributes["new_contactnumber"] = customer.GetAttributeValue<String>("new_contactnumber");

the "contactnumber" was succes to retrieve contact number from parent entity. But for the "gender" i only get the value. Like '10000000'. How can i retrieve the label of optionset from parent entity?