Gotogate Refuses to Refund My Voucher After Airline Cancels Flight

I live in Istanbul and purchased my plane ticket through Skyscanner via Gotogate. My flight with Kuwait Airways, scheduled for June 18, was canceled by the airline through absolutely no fault of my own, and the transportation service I paid for was never provided at all.
For this booking, I used a voucher worth $889. I also made additional payments by credit card: a $410 fare difference fee, a $137 seat reservation fee, a $23 special diet fee, and a $17 check-in fee. My Gotogate booking reference numbers are 84M3F9 and 1126-970-344.
After the cancellation, I both corresponded with and called Gotogate. They first created a new reservation, then canceled that one too. After that, they told me they could issue a refund, and I agreed to it. But when I asked about the total amount I'd be getting back, they told me only the $410 fare difference fee, the $137 seat reservation fee, and the $23 special diet fee would be refunded — and that the $889 voucher amount would not be refunded at all, since it was a one-time-use voucher. Because of this, I have not yet approved the cancellation.
Given that the flight was canceled by the airline and the service was never provided at all, it goes against both fairness and reasonable consumer expectations for such a large voucher amount to simply be forfeited, with no refund issued and no replacement voucher provided. I'm requesting that all amounts I paid be refunded to me without any loss on my part — the credit card payments refunded to my credit card, and the $889 voucher amount either reissued as a new voucher or made usable for me in some appropriate way.