Best way porting linux software written in C Gtk Ecplipse to C++ Qt Qt Creator?

[anda_skoa]

It should be possible to import a makefile base project into QtCreator, but also to use Eclipse for the Qt version.

One migration technique when coming from a different UI technologie is to
- first remove the old library from the build system, in your case GTK
- then #ifdef everything that does not compile anymore
- then work from main() down, porting each #ifdef'ed part as you encounter it

Rewriting from scratch has the risk that you could miss things that the old applications was able to do or make mistakes when implementing processing logic.
Especially since this is, as you write, a complex application.

Cheers,
_

If this is something that a company needs to have done, I would even suggest to at least get an offer from one of the Qt service providers who have many years of experience with these kind of migrations.
Some of them even do such estimates for free.