GamingCuracao doesn't have an official complaints portal on their website like other curacao license providers (ex. Curacao e-gaming). That email address is useless as they never answer any email you send to them, I've tried in the past with other bookmakers.
Do you mind to give it another shot? Perhaps adding as many details as you can this time, and specifically adds that you've tried couple of ADR to no avail, and that they suggest you to escalate to the licensor.
Previously, there are other [relatively] good arbitrator aside from AG and CG, the Pogg, but I tried to access their site not long ago for another case I am overseeing and it seems they rebranded themselves to casino reviews. I am not sure if the rebrand followed by an improved dispute resolution or not, but in the past, IIRC, the Pogg operates in a bit untransparent manner.
Not in a term like they're taking sides, but rather their investigation is not available for public to consume. They get an complaint email, they reach the casino, they made internal and private investigation, and come back with a result.
You're free to try this one, if you think going to GC is not the best option for the time being.