Having found Gammu usage in proprietary software and finding somebody who is actually having the source code, the only remaining (and the hardest) step was to make them publish it.
I started with explaining what actually GPL means, giving them links to useful documentation what they should do, but I'm probably not good enough in explaining the things.
All I was given is source of some parts of Gammu code they have used (it is available in our git repository since then). Any attempts on explaining them that they should publish full source code of the thing to comply with GPL have simply failed.
The problem with the sources they have given is that it is hardly to reuse in Gammu - it is hard to find on which version they based and more importantly, it really does not include much code which would be useful in Gammu these days.