GMCs (Guaranteed Mortgage Certificates) Love Machine 14 years ago GMCs (Guaranteed Mortgage Certificates)GMCs represent a portion of undivided interest in conventional loans and similar participations acquired earlier by Freddie Mac. The first GMC was issued in 1975 by Freddie Mac.Recommended for you:Government National Mortgage Association (Ginnie Mae)Conventional MortgageGuaranteed Insurance ContractMortgage Pool