(Did I mention IANAL?)

Yeah, but at least in the U.S., the rules are that if you made something subject to copyright protections as part of your job, absent any specific agreement between you and your employer, it's "work for hire" and thus owned by your employer. That is, it doesn't require any specific agreement/contract to make it your employer's. It requires a specific agreement to make it yours and not your employer's.

(Unless you didn't write the "custom functionalities" as part of your job, which can be pretty dicey. If you did it during off hours but using your employer's computer, for instance, it becomes a difficult question to answer whether you wrote them specifically as part of your job. Even if you did it during off hours and on your own hardware with your own license for the proprietary software in question, it'd be difficut to decide whether it was work for hire or not.)

So, if you want to do things the right way, that involves making an agreement with your employer that you own the copyright on the code in question. If you moved forward with distributing the software you wrote without such an agreement that wouldn't be "doing things the right way." That would be "hoping you don't get caught." I don't think there'd really be any confusion, really, since it's pretty cut-and-dried that unless you've discussed this with your employer and made an agreement, you clearly don't have the right to distribute this software under any license (open source or otherwise.)

Now, even the step of making such an agreement would be kindof complex. Assuming your employer was open to the idea, you might assume you could make a contract transferring ownership of the work to you, but contracts require "consideration", meaning both parties (you and your employer) would have to benefit from it. You could potentially argue that by open sourcing it, your employer has the chance of benefitting from voluntary contributions by a community of Open Source developers, and that could likely (IANAL) fulfill the consideration requirement.

Another option, and I'm thinking probably the most straightforward, would be for your employer to retain the copyright and grant you permission (and I think it'd be good to get that permission in writing, though it wouldn't have to be on paper; an email would be fine, but you'd probably want to forward it to an email address you own and would retain if you ever left the company) to publish it as an open source project (and I believe the GPL would be fine for this) on their behalf as part of your job. Lots of companies do this. Netflix for instance.

And, again, IANAL and this isn't legal advice. I've done some research and I'm interested in these topics, but I'm not an expert and you'll get a lot more mileage from consulting a lawyer.