Need to Bring Back most Recent Departure Date from Sales table for Customers in Recipient table
If the only two references from the table GccTbl_Sales are the columns iCustomerId and tsDepart, this should work:

SELECT DISTINCT

iRecipientId AS RecipientID, NmsRecipient.iCustomerId AS Cust_Num, sFirstName AS FirstName, sLastName AS LastName, sEmail AS Email,
iBlacklistPostalMail AS NoMail, iBlacklistEmail AS NoEmail, iBlacklistMailGCCL AS DontMailGCCL, iBlacklistGCCL AS DontEmailGCCL,
iBlacklistMailOAT AS DontMailOAT, iBlacklistOAT AS DontEmailOAT, sLastBrand AS LBT,sFrequencyDetail AS Frequency, sRecency AS Recency,
GccTbl_HouseholdScores.iNumTripsBkd AS Trips, GccTbl_Sales.tsDepart AS Departure_Date

FROM (neolane.NmsRecipient

INNER JOIN neolane.GccTbl_HouseholdScores ON NmsRecipient.iRecipientId = GccTbl_HouseholdScores.iHouseHoldId)
INNER JOIN (
            SELECT GccTbl_Sales.iCustomerId, MAX(GccTbl_Sales.tsDepart) AS tsDepart
            FROM neolane.GccTbl_Sales
            GROUP BY GccTbl_Sales.iCustomerId
            ) AS GccTbl_Sales ON NmsRecipient.iCustomerId = GccTbl_Sales.iCustomerId

WHERE iIrateFlag= 1
    AND iOperationalFlag= 1
    AND iEmployeeFlag= 0
    AND iDeceased= 0

ORDER BY iblacklistPostalMail,iBlacklistEmail,iBlacklistMailGCCL,iBlacklistGCCL,iBlacklistMailOAT,iBlacklistOAT;


And if the DISTINCT was put there in order to remove the duplicates of iCustomerID, I think you're safe to remove it now.