Nubel stormed out of his penalty area on 66 minutes and landed a kung-fu kick on the chest of Gacinovic.
The 23-year-old Schalke captain was shown a red card for the challenge, while Gacinovic was substituted but escaped with bruised ribs.
On Monday, Nubel said his apology to Gacinovic had been accepted.
"I thought that was awesome," Nubel said.
Nubel was sent off in a Bundesliga match against Borussia Monchengladbach in early 2019.
While there have been six-month bans for attacks on referees in the past -- Hertha Berlin's Levan Kobiashvilli for attacking Wolfgang Stark in 2012 -- this had been expected to be one of the longest in Bundesliga history for a foul.
The record stands at 10 weeks for former Germany keeper Uli Stein, who, as a Hamburg player, infamously punched Bayern Munich forward Jurgen Wegmann in the 1987 German Super Cup final.