What Can I Do?

Trade for one.

Unfortunately, the best thing you can do is find someone who owns an extra Meteor Mash Metagross and try and get them to trade with you.

I suppose an email to Niantic about the issue wouldn't hurt, but don't get your hopes up.

Have a beer and forget your troubles

I had the same problem: I evolved a lucky Metagross after the start of the community day and he had eartquake. Sending an email to niantic you will not receive any reply probably

Use it as a psychic attacker?