I had this topic in mind for quite a long time, but it got more recently more attention because of discussions on gammu-users mailing list. Gammu should use some library for do basic stuff like strings or date manipulations, logging and so on.
The reason is that having own implementation of all that is not really useful and just source of another amount of bugs. Currently mi intention is to use GLib, simply because I don't know any C alternative (though there are probably lot of C++ cross platform libraries).
Of course the biggest question is how to make the change, because it will be obviously intrusive and making such big changes will probably lead to several months of work without really making another release. Even if it will be done on separate branch (what would be most likely the case), developers will spend most of time on this issue and not much time will be spent on bug fixes (and backporting them).
Hmm I don't know the gammu system pretty well, but if it's not a library, it should be pretty clean to add GLib to make use of generic functions because most of its functions take standard arguments, not specific types (like it happens usually for C++). I have converted feng (from lscube.org) to glib in a matter of a couple of weeks, alone.