I believe that it is a 2-of-2 MultiSig... so it would appear you have access to required private keys.
As far as I can tell, the problem is that you simply cannot create the
SegWit MultiSig address from the
Legacy MultiSig address any more... as noted in that link, the "createwitnessaddress" command was removed from Bitcoin Core (so it also doesn't exist in BitcoinGold Core)...
Without that functionality, I don't know of any easy way to "convert" the legacy MultiSig (P2SH) address to a SegWit MultiSig (P2WSH-over-P2SH)...
However, what I've just tried is using Bitcoin Core 0.13.0 (The last version of Bitcoin Core with the "createwitnessaddress" command, it doesn't even need to be synced!)... and it seems to create the same P2WSH-over-P2SH... using "createwitnessaddress".
ie. from block.io:
* Address: 35x8Y6r9e4hnwPQ9y6RgEzgvCZ2urLmJJA
* Address Type: P2WSH-over-P2SH
* Required Signatures: 2
...
* Public Keys: ["02b5a89a361fe7cc2aea909700f11a53c72fce3d1f10ccb9411d943dd2e6b00646", "03e8086a8f5160d161536557b02f24f49644d414b4d5ea43e4f9f6a596d655e31b"]
Using Bitcoin Core 0.13.0:

createmultisig 2 '["02b5a89a361fe7cc2aea909700f11a53c72fce3d1f10ccb9411d943dd2e6b00646", "03e8086a8f5160d161536557b02f24f49644d414b4d5ea43e4f9f6a596d655e31b"]'

{
"address": "3GhvcWciiLjhH4avpBhpU749W5qXZmLTRe",
"redeemScript": "5221..........52ae"
}
createwitnessaddress 5221..........52ae
{
"address": "35x8Y6r9e4hnwPQ9y6RgEzgvCZ2urLmJJA",
"witnessScript": "0020..........8f1a"
}
Theoretically, one should then be able to use BitcoinGold Core, the redeemScript, the witnessScript and the two private keys from block.io to manually create a transaction and sign it (following
the instructions here: Step 4 - Spending the MultiSig)
Unfortunately, I don't have any BitcoinGold to try and test my theory...
So I can't guarantee it will work... but it shouldn't cost you anything to try, except a bit of time