Lot of people contributed to Gammu, python-gammu and Wammu in the past. You can find the most visible here.
Michal has started to contribute somewhere in 2002 with improvements to AT module and later contributed module for Alcatel phone. Later he wrote Python bindings for Gammu and GUI application on top of it. In 2007 he has become maintainer of the Gammu project and he is still in the charge here.
Marcin has originally started the project as fork of Gnokii because he was not happy with some parts of the existing code. He did contribute most of core and Nokia modules and he did lead the project till January 2007, when he resigned because lack of time. He is not active in the project anymore.
As the code orignally started as the fork of the Gnokii project, we would like to thank all contributors there, especially Pavel Janík, Pawel Kot and Manfred Jonsson.