First question is, of course, if this former employee is still emailing people with your company email address. If he is, you might be able to hold him liable for this, since he's no longer employed by your company.
You might also want to get some legal advice and file an official complaint to Google to ask Google to close this GMail account because he used it for your company. Now, with him gone, the account should be closed too, (or handed over to you) else customers might contact the wrong person. For this to succeed, a judge would have to approve this request and it will cost you some.
I would advise you to not allow other employees to use their GMail account for your company, unless they are willing to transfer those accounts over to the company once they leave the company. Using GMail should not be a problem, not if it's used for your company or just private usage. But once the two are mixed, your company will need a specific policy which will be enforced.
But if this former employee has removed the link between his account and your company and is just behaving nicely, there should be no problem with him keeping his account and you shouldn't really be bothered about this case.
I would advise you to set up a policy for using GMail accounts that are linked to company accounts! Tell your employees to either remove those links or be willing to give up their accounts if they ever leave your company. By using GMail for your company, it actually becomes a company resource, not a private resource.
It's just that the related costs when it comes to litigation tends to be expensive and success isn't guaranteed.