There are several test suites for testing encrypted storage with GCM cipher mode:
- sharding_ese_gcm.yml
- replica_sets_ese_gcm.yml
- aggregation_ese_gcm.yml
- core_ese_gcm.yml
Those suites assign 'AES256-GCM' value to the TestData.encryptionCipherMode , but the mongo shell ignores it and starts mongod without --encryptionCipherMode parameter.
Thus mongod uses default CBC cipher so it wouldn't be testing GCM at all.