On the last tour, they opened with two Kiske songs and closed with two Kiske songs, everything in between was Deris', it comes as no surprise for me to read that the's happy to let Kiske sing his own songs again.
20 years of busting his ass off through records and album shows, after having saved a dead band back in 2004, and to listen everywhere how Kiske was better for those song... I can only imagine how annoying it must have been.
I don't know if the reunion is permanent, but where could they go from here? they've been around since forever, they're not old but they play a kind of music that requires a lot of intensity (can you imagine them at 65 playing Eagle Fly Free?), it would be kinda plain to go back to the regular lineup after that.
I think it will play out like the Dio reunion with Black Sabbath, they will do the tour and if they get along fine, they will consider a new album all together. But I assume that 2019 will be Gamma Ray's year as well, they will have a break and do something else in 2020.