Gündogan has been out of action since suffering a recurrence of a back injury in BVB's 4-0 victory over FC Augsburg on the opening weekend of the current campaign, while Schmelzer injured his calf in Germany's friendly win over England at Wembley Stadium in November.
Returning to full strength
The midfielders and , who both picked up ankle knocks in Dortmund's defeat to Bayer 04 Leverkusen last month, also trained with the rest of the squad.
Meanwhile, first-choice centre-backs and worked on their fitness in the weights room, with the former expected to return to first-team training soon. "We will try to bring him back to full fitness over the next two weeks," said Klopp.