diff --git a/cryptography/hazmat/bindings/commoncrypto/common_cryptor.py b/cryptography/hazmat/bindings/commoncrypto/common_cryptor.py
index e8b6cf9..8f03bc3 100644
--- a/cryptography/hazmat/bindings/commoncrypto/common_cryptor.py
+++ b/cryptography/hazmat/bindings/commoncrypto/common_cryptor.py
@@ -62,6 +62,7 @@
     kCCModeXTS = 8,
     kCCModeRC4 = 9,
     kCCModeCFB8 = 10,
+    kCCModeGCM = 11
 };
 typedef uint32_t CCMode;
 enum {
@@ -98,6 +99,10 @@
 """
 
 CUSTOMIZATIONS = """
+// Not defined in the public header
+enum {
+    kCCModeGCM = 11
+};
 """
 
 CONDITIONAL_NAMES = {}