A DESCRIPTION OF THE PROBLEM :
Tested OpenJDK 13 EA with SoftHSM2 v2.5.1 (current development version). Calling PKCS11.C_EncryptInit ends always with the error CKR_ARGUMENTS_BAD. The following code block in SoftHSM.cpp is responsible for this error:

		case CKM_AES_GCM:
			algo = SymAlgo::AES;
			mode = SymMode::GCM;
			if (pMechanism->pParameter == NULL_PTR ||
			    pMechanism->ulParameterLen != sizeof(CK_GCM_PARAMS))
			{
				DEBUG_MSG("GCM mode requires parameters");
				return CKR_ARGUMENTS_BAD;
			}

It seems that PKCS11.C_EncryptInit() does not forward the CK_GCM_PARAMS correctly to the underlying Cryptoki library.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
No error.
ACTUAL -
Got error sun.security.pkcs11.wrapper.PKCS11Exception: CKR_ARGUMENTS_BAD

---------- BEGIN SOURCE ----------
  public static void main(String[] args) {
    try {
      final String pkcs11ModuleName = "/usr/local/lib/softhsm/libsofthsm2.so";
      final String functionList = "C_GetFunctionList";
      final boolean omitInitialize = false;
      PKCS11 pkcs11Module = PKCS11.getInstance(pkcs11ModuleName, functionList,
            null, omitInitialize);
      long slotId = pkcs11Module.C_GetSlotList(true)[0];
      
      //final long CKF_RW_SESSION     = 0x00000002L;
      //final long CKF_SERIAL_SESSION = 0x00000004L;
      long flags = PKCS11Constants.CKF_SERIAL_SESSION | PKCS11Constants.CKF_RW_SESSION;
      long hSession = pkcs11Module.C_OpenSession(slotId, flags, null, null);
      char[] pin = "123456".toCharArray();
      pkcs11Module.C_Login(hSession, PKCS11Constants.CKU_USER, pin);

      // generate an AES key
      CK_MECHANISM pMechanism = new CK_MECHANISM(PKCS11Constants.CKM_AES_KEY_GEN);
      List<CK_ATTRIBUTE> attrs = new LinkedList<>();
      attrs.add(new CK_ATTRIBUTE(PKCS11Constants.CKA_CLASS, PKCS11Constants.CKO_SECRET_KEY));
      attrs.add(new CK_ATTRIBUTE(PKCS11Constants.CKA_KEY_TYPE, PKCS11Constants.CKK_AES));
      attrs.add(new CK_ATTRIBUTE(PKCS11Constants.CKA_VALUE_LEN, 16));
      attrs.add(new CK_ATTRIBUTE(PKCS11Constants.CKA_SENSITIVE, true));
      
      long hKey = pkcs11Module.C_GenerateKey(
                    hSession, pMechanism, attrs.toArray(new CK_ATTRIBUTE[0]));
      
      byte[] iv = new byte[12];
      new SecureRandom().nextBytes(iv);
      byte[] aad = "hello".getBytes();
      CK_GCM_PARAMS params = new CK_GCM_PARAMS(128, iv, aad);
      pMechanism = new CK_MECHANISM(PKCS11Constants.CKM_AES_GCM, params);
      
      pkcs11Module.C_EncryptInit(hSession, pMechanism, hKey);
    } catch (Exception ex) {
      ex.printStackTrace();
    }
  }
---------- END SOURCE ----------

FREQUENCY : always