As discussed on the other thread referenced, if your company created the entire GPL library, and still owns the entire copyright in it, it does not restrict your company in any way -- it's simply a permission your company has given others.
In your case, there are other authors. If all of those other authors indeed licensed their copyrights under permissive licenses, you may only need to comply with those licenses. However, if there are contributors who contributed to your GPL library who did not state an explicit license, you may have a problem -- you should have contributors sign a CLA, or at least sign a DCO and state an explicit license. Presumably, that license would be the GPL, but that would not be something I can tell you. The details here can get complicated, so if you don't know you have permission to use all of your code, consult an attorney directly. The point is, if you are taking any contributions under a non-permissive license, you might be required to comply with the GPL provisions you're trying to avoid.
"Using" the GPL package is entirely permissive -- copyright does not restrict use at all. Presumably, you're distributing the GPL library, and interacting with it in some way more interesting than "mere aggregation." The details of GPL compliance are, again, complicated. To oversimplify, linking is not allowed, but some "distant" interactions are allowed. Again, if this information isn't enough to answer your question, consult an attorney directly.