Can I use a free software/open source license to my GD project? If yes, what license do you recommend? Is a Creative Commons license usable in this situation?
How can it be possible if, for example, a GPL license is chosen? Should I include the copyright and license notes in the top of the source .gdg file (which seems to be a sort of XML file)?
I know that’s a lot of questions, but I thank for the considerations in advance. 
Yes, you can choose any license for your projet: If you share it, include a Readme file with the license and it should be ok 
You can also add a comment in the events of your game to tell the reader to refer to this Readme file
Sorry for my bad english,
You can do what you want of your GD project.
If you use a GPL licence, you should give the sources, not with a Creative Commons licence.
I recommend NOL licence (No Obligation Licence), you do what you want.
Or write your own licence.
Thank you for the useful replies! 
I think now its clear that the “event comments” work the same way as traditional “code comments” commonly used to store license information.