I'm a grad student in the US, so take what I say with a grain of salt, but if I were you in my gap year, I'd be thinking about the following:
Did you do any research for your senior thesis/MA? If so, turn it into an article you can publish.
Build connections/collaborations with your intended research site. Do whatever pilot work you would otherwise have done in your first summer of graduate school (building connections, participant observation, etc.).
Apply for fellowships you qualify for as a recent graduate/ incoming grad; not sure what those would be Europe, but seek external funding whenever you can.
Are there any methods you plan to use / feel you need to learn/strengthen for your research? Develop what you can now. I found it useful to learn good interviewing skills through volunteering at a clinic and helping w/ an ongoing qual. research; stats skills through helping analyze different datasets, etc.
Maybe submit something to a conference or two (apply for travel awards!), attend relevant workshops/summer institutes.