The 31-year-old has signed from relegated side SV Wehen Wiesbaden.
Despite only rejoining the club last summer and having a contract until 2023, Zieler was unceremoniously told he would play no further part in Kenan Kocak’s plans in the upcoming season.
The 26-year-old left Der Club earlier in the summer after becoming surplus to requirements, and has now signed a two-year contract with the Lower Saxony club.
Rapp will spend a further season on loan with Darmstadt from Union Berlin; Führich joins from Borussia Dortmund.
Klewin arrives from 2. Bundesliga champions Arminia Bielefeld whilst Gnjatic was previously with Polish Ekstraklasa side Korona Kielce.
The season will start on the 18th September with two rather than the usual one game, with the Bundesliga starting the same night.
He will spend the next season at the HDI-Arena having fallen out of favour under current Effzeh coach Markus Gisdol.
The 24-year-old, who has previously had loan spells with Holstein Kiel and Jahn Regensburg in the 2. Bundesliga, has signed a two-year contract with die Lilien.
Both had been training with the first team since the restart in May, and each has now been handed a two-year contract.
The attacker returns to the club from Werder Bremen after 13 years away.