The API is working great now!! On the last version of WHMCS, the payments that our clients sent to us through CoinGate weren't being confirmed by WHMCS so we had to do it manually.
In that occasion, we investigated that issue with CoinGate API but didn't find anything bad configured, so that was probably an issue WHMCS had with their API..
Now in the new version of WHMCS, this is working perfectly! It confirms the payment automatically and very fast, almost instantly, so we are having a good experience with CoinGate so far!