The Germany international was limited to just three appearances last season due to a back injury, an issue that ruled him out of the FIFA World Cup in Brazil.
Gundogan, who moved to Signal Iduna Park from Nuremberg in 2011, underwent surgery on his troublesome back in June and targeted a return in September.
The 23-year-old has travelled to Kirchberg, Austria, where Dortmund are partaking in a pre-season training camp.
And though he is not yet ready to return to competitive action, Gundogan is delighted to be back among the group.
"Finally I am back. I am very happy," he told Bild.
"Even if I can not work with the team, it is just great to be [with] colleagues.
"I will exercise in the pool and ride a bicycle. [It is] not clear how long it takes before I can take the next step."
Dortmund will remain in Austria until Saturday, as they step up preparations ahead of their Bundesliga opener with Bayer Leverkusen on August 23.