He's a good player. I wanted Gladbach to sign him way back when he left Bremen, and he's developed well at Hoffenheim since then. Also makes sense for TSG as, not only is Grillitsch out of contract next summer, but they've got a gaggle of good midfielders & needed to open playing time for their younger talents with Rudy's return.
As on top of Samassekou, they have Dennis Geiger ('98), who is a NT-caliber young-midfielder, and they've picked up Angelo Stiller ('01) from Bayern...who's a very highly rated kid in need of consistent senior level PT and picked them because TSG's current coach used to be Bayern's youth coach.
Could be a really interesting year for them as Kramaric is also expiring next summer & getting linked to Serie A (AC Milan). They have a lot of interesting young talents...on top of Geiger & Stiller; Bogarde ('02), Baumgartner ('99), John ('01), Rutter ('02), & Beier ('02) are really interesting U21 players...but Kramaric has been an absolute talisman for them since arriving from Leicester.