Sorry for being silent, I had just forgotten about this forum thread. I hoped to get some approval from minergate, they certainly know about the meaning of that field (which I called just "mysterious"). Thanks for publishing the patch.

Here's my solution:

diff --git a/src/cryptonote_core/tx_extra.h b/src/cryptonote_core/tx_extra.h
index 93d2e21..6f5fbe4 100644
--- a/src/cryptonote_core/tx_extra.h
+++ b/src/cryptonote_core/tx_extra.h
@@ -38,6 +38,7 @@
 #define TX_EXTRA_TAG_PUBKEY                 0x01
 #define TX_EXTRA_NONCE                      0x02
 #define TX_EXTRA_MERGE_MINING_TAG           0x03
+#define TX_EXTRA_MYSTERIOUS_MINERGATE_TAG   0xDE

 #define TX_EXTRA_NONCE_PAYMENT_ID           0x00
 #define TX_EXTRA_NONCE_ENCRYPTED_PAYMENT_ID 0x01
@@ -158,14 +159,24 @@ namespace cryptonote
     }
   };

+  struct tx_extra_mysterious_minergate
+  {
+    std::string data;
+
+    BEGIN_SERIALIZE()
+      FIELD(data)
+    END_SERIALIZE()
+  };
+
   // tx_extra_field format, except tx_extra_padding and tx_extra_pub_key:
   //   varint tag;
   //   varint size;
   //   varint data[];
-  typedef boost::variant<tx_extra_padding, tx_extra_pub_key, tx_extra_nonce, tx_extra_merge_mining_tag> tx_extra_field;
+  typedef boost::variant<tx_extra_padding, tx_extra_pub_key, tx_extra_nonce, tx_extra_merge_mining_tag, tx_extra_mysterious_minergate> tx_extra_field;
 }

 VARIANT_TAG(binary_archive, cryptonote::tx_extra_padding, TX_EXTRA_TAG_PADDING);
 VARIANT_TAG(binary_archive, cryptonote::tx_extra_pub_key, TX_EXTRA_TAG_PUBKEY);
 VARIANT_TAG(binary_archive, cryptonote::tx_extra_nonce, TX_EXTRA_NONCE);
 VARIANT_TAG(binary_archive, cryptonote::tx_extra_merge_mining_tag, TX_EXTRA_MERGE_MINING_TAG);
+VARIANT_TAG(binary_archive, cryptonote::tx_extra_mysterious_minergate, TX_EXTRA_MYSTERIOUS_MINERGATE_TAG);

If that new tag/field consistently parses all of these txes, then I think it's a good solution. Feel free to put it on github, maybe others will have comments.

It'd be nice to get this on github. If you don't want to do it, I can do it. If I do, what author information would you like ? Just blashyrkh ? Another name ? An email address ?