It's clearly (b) from the context.

I don't know about Yahoo, but in other organizations I've seen employees using gmail, for work, because it's more convenient than the corporate/enterprise email platform.

Sure but it's usually against corporate policy and reason to fire the person. This has happened in companies I have worked in the past. People take print outs and take them outside office. This is against company policy for confidential documents).

What kind of companies are these? I have worked in a dozen companies so far and in not one of them using GMail for personal use was against the corporate policy.

GP clearly says "gmail, for work".

I think a vast majority of companies would fire (at least, call it out as against policy) if you just went ahead and used gmail for work emails because the official one is crappy.

Aol, at least, migrated their corporate email to gmail several years ago. Before that, it was exchange.