Sammy Studios have published many titles in the Guilty Gear franchise, so when they bought Sega in 2004 it became unclear if they still owned the rights.
The developer, Arc System Works, have recently self-published some Guilty Gear titles, although they don't feature many old characters and have completely different gameplay styles.
In an interview with Gamasutra, Guilty Gear designer Daisuke Ishiwatari claimed that Sega do indeed still own the rights:
If Sega do indeed own the rights, it would be good if they published some new, traditional Guilty Gear games in HD, à la King of Fighters XII and Super Street Fighter II Turbo HD Remix.
Gamasutra: This is a tough question, I don't know if you can answer it, but there have been some rumours on the 'net about Guilty Gear and Sega getting the rights, which is why [Arc System Works' Dynasty Warriors-style action title] Guilty Gear 2: Overture didn't feature too many older characters — and stuff that I don't really understand. Though you might not be able to provide any answers...
Ishiwatari: Right, I might not be able to provide answers there.
Gamasutra: You know, rumours... on the Internet...
Ishiwatari: Yeah, I can't really talk about it much... except to say that they're basically true. (laughs)