Not a lawyer, but your company probably owns your code (this is typical).
You do not have permission to release it to the world. At the very least your employer can justifiably terminate your employment based on this -- and you have to be careful that you don't include any non-GPLv3 code with the GPL parts.
If the code is GPLv3, then if that code is distributed (sold or given) to a customer than the customer is legally entitled to the GPLv3 portions of the source code and they can distribute the code however they choose.
If you have GPLv3 software as a service, e.g., your company can keep modified GPL code themselves, but never release/distribute it to anyone. Then they can keep the code secret and only use it in-house.