As revealed by Eve Myles herself on her Twitter page! I feel it's a little bit late to be recording today and have the release next month so I doubt this is for 2.6 (but I may be wrong) so possibly a solo Gwen story opening up Series 3?
I remember saying maybe there was a misunderstanding from those tweets but many people were convinced it definitely meant she didn't want to play Gwen again at all. I am not sure what those tweets actually meant but I think this shows you should not jump to conclusions until official statements are given. And no random tweets on twitter are not official without clarification.
Anyways I am glad she's back and the interpretation behind those tweets were either wrong or she changed her mind.