The Robins confirmed the appointment of former Reading boss Steve Coppell as their new manager on a 12-month rolling contract on Thursday, but Millen will remain in charge for the last two games of the season.
Winger Campbell-Ryce missed last weekend's 3-0 defeat at Scunthorpe due to a hamstring injury but is fully fit again.
Goalkeeper Gerken has recovered from a collarbone injury to put himself back in the frame, but with Stephen Henderson impressing during his absence he could struggle to force his way back in.
Meanwhile, long-term absentees Jamie McAllister (calf), James Wilson and Jamie McCombe (both knee) remain on the sidelines.
