Problem is here: https://github.com/stellar/java-stellar-sdk/blob/master/src/main/java/org/stellar/sdk/responses/TransactionResponse.java

Problem is:

  @SerializedName("source_account")
  private final KeyPair sourceAccount;

When it want to serialize KeyPair, it scream with exception about secretSeed cannot be serialized and converted into JSON.

This problem is in all responses with source_account property. Therefore currently I have to duplicate all these classes by hand, remove source_account property.

And every response it needs to iterate over all objects and transform them by hand just because of this:

for (TransactionResponse transactionResponse : transactionResponsePage.getRecords()) {
                GetTransaction transaction = new GetTransaction();
                transaction.setCreated(transactionResponse.getCreatedAt());
                transaction.setFee(transactionResponse.getFeePaid());
                transaction.setTxHash(transactionResponse.getHash());
                transactions.add(transaction);
}

Why GSON? Why don't you use Jackson.