I don't think it would be a derived work unless there is a 1:1 correlation between lines of code in your thing and the open source code. We're not talking about a patent here, where the concept of the invention is important.
If it is only used internally then it doesn't have to be.
You could never be certain that it doesn't accidentally leak out or get shared or included in another project.
You should try to work with OpenPGM to make the Erlang interface that you need; then it is open source, other people may help maintain it for you, you get a free code review