There is a discussion forum on GitHub, where you can discuss with other community members and share your ideas.
You can also ask related questions on Stack Overflow.
The main list where the discussion was being held was gammu-users@lists.sourceforge.net. It's just generic place where users or developers can discuss anything related to Gammu (this includes Wammu or third party frontends like Kalkun).
In past, there used to be some problems with licensing of some code in Gammu and separate lists for this purpose has been created - gammu-legal@lists.sourceforge.net. It is not much used nowadays, but should you find any legal problems with Gammu, please use this mailing list.