Additionally, unless your work environment is sophisticated enough to fiddle with the certificates on your machine, and run a MITM proxy, you should be safe using something like GMail over https. Now I'm sure some companies do manage to intercept outgoing https traffic, but I doubt that most companies do.

OTOH, I still think being paranoid is the safest policy, so if you're plotting to overthrow your boss, or sell secrets to your biggest competitor, I still wouldn't do it on the company networking, and/or using a company computer.

If your Gmail account is through work (i.e. not a personal one) they already have access to it.

Agreed. I was referring to the parent post's comment about using personal accounts at work. That might be safe, to some extent. But again, I would lean on the side of paranoia if you're talking about anything that could get you (fired|put in jail).

In any case, if you would plan any of that, I would guess that it would most probably leak out through entirely nontechnological channel - e.g. by somebody overhearing in person, or even by a co-conspirator defecting for their own gain.

Cloak-and-dagger games are very similar to building your own crypto: likely to be broken at a fundamental level, never mind the amount of magic security glitter that you pile on top.