What's best for everyone involved, including yourself, your career, and the company employing you:
Read the source code. Find out how the source code works. When you know how it works, write it yourself. Without seeing the original source code. That way you are not copying anything, so you are not committing copyright infringement.
If you just take some GPL licensed code and add it to your project, then you have the choice: You license your project under the GPL license, or you admit that you committed copyright infringement and can be sued. If the copyright holder of the GPL licensed source code finds out (how could that happen? Well, maybe your colleague calls them and tells them), not only is your company in trouble, but your career is in big trouble. So you will never do this without talking to your company's lawyer (unless it is common knowledge in your company that the whole product is GPL licensed).
But even if the product is GPL licensed: Many companies use "dual licensing". I can download their source code for free under the GPL license, or I can get a license with full product software for lots of money. If the company does that, they can't just use someone else's GPL licensed code, because then they are not allowed to do dual licensing anymore which kills the business.
So: Don't do it unless you talk to your lawyers. And very often lawyers are expensive, so your boss will tell you not to do it, because it's cheaper you write it yourself, than copying and getting your lawyers involved.