https://support.google.com/mail/contact/abuse

If they can't figure out the problem from there with you description then they are just incompetent and you shouldn't be doing business with incompetent companies no matter how large or popular they are.

You should bring everything you rely on in house as much as possible if not possible then on business that has an incentive to work with you. If you can't afford to then question if your product is really providing value. Why anybody with greater memory than a goldfish would build on top of a google service is beyond me (probably just people falling for ads or propaganda) it's no different then building on known vulnerable software.

Fool me once, shame on you. Fool me 297 times (+ all the soft killed services without adequate support) , shame on me!