Unless you have specific reasons to discuss an issue with a particular person in private, please use one of the mailing lists for Gwyddion discussion.
Anyway, the official contact (whatever it means) and leader of scientific part of the project is Petr, microscopy-related questions should be directed to him. Bugs should be reported to him.
On the technical side the project is being led by David. Should you have porting, packaging, translation, and inner working of things questions, he's the right one to ask.
See also file AUTHORS in source code for responsibilities,
and areas of expertise.
Petr Klapetek
Czech Metrology Institute,
Okružní 31, 638 00, Brno
Czech Republic
klapetek <at> gwyddion <dot> net
David Nečas (Yeti)
Department of Physical Electronics
Faculty of Science
Masaryk University
Kotlářská 2, 611 37, Brno
Czech Republic
yeti <at> gwyddion <dot> net
Gwyddion mailing lists are hosted on SourceForge, see SF Gwyddion mailing list page. You have to subscribe to post. There are two lists:
gwyddion-users where use and microscopy generally can be discussed. You can subscribe on gwyddion-users SourceForge page.
gwyddion-devel where development issues are discussed. You can subscribe on gwyddion-devel SourceForge page.