The 25-year-old Germany international is under contract until June 2017 at Signal Iduna Park after penning an extension in July following intense speculation over his future given links to clubs in Spain and England.
Talks are now on-going to pin Gundogan down to a longer-term agreement, amid speculation linking him with a host of Europe's leading clubs.
"The importance of Iklay is undisputed - I would like to tell you that he has extended until 2026," Tuchel said at a pre-match news conference ahead of Saturday's Bundesliga match against Hannover.
"Unfortunately that is not the case, but we are in talks. We have a good chance.
"The interest in Gundogan is great, which is why talks are not very easy."
Meanwhile, top scorer Pierre-Emerick Aubameyang sat out Dortmund's final training session ahead of the match with the Bundesliga's bottom club.
Aubameyang scored his 30th club goal of an incredible campaign in Tuesday's 3-1 DFB Pokal win at Stuttgart but suffered a knock and will be assessed before kick-off against Thomas Schaaf's men.
"Aubameyang has taken a hit and was still being treated as we waited for today's training," Tuchel explained, before urging his players to ignore Hannover's lowly standing.
"The preparation for an opponent is impartial of their position in the table," he added.
"Tuesday was tough but the batteries are recharged again. Auba got a knock, we have to see about that.
"It's hard to adjust for Hannover as their line-up and style of play will be hard to predict. Our aspiration is to dominate the game."