Most of the Edited Manga groups in Spanish do the same thing. They just clean the easy texts in English (only if they are in a bubble), put away the credits page in English and then add their own credits page. Even one of my works (Paradise Residence ch1) was targeted by them, the funny thing is that I make dual releases of that series in Spanish and English. Oh boy... that people is so dumb. Of course, there are also honest groups in Spanish who clean their own raws, but they also suffer from the same stealing, so they have to add a water mark on each page in order to make them desist.
Anyway, back to Gantz, that's one of the most worked series by international groups. The most popular page in Spanish where they lurk is SubManga. This is the section for Gantz:
http://submanga.com/manga/Gantz
Gee, 27 groups doing the same thing! That's sick...
Just check them out, there may be more groups stealing your work.
Ciao!