At this point I have reviewed the only video i can find of the time of his hand signal.
The point in time is when he is seated, awaiting his turn for the leak-check of his suit, (helmet and gloves will be fitted, hence his skull cap is ready, and his blue ventilation box is by his side).
His commander (Sergei Prokopyev) is undergoing the test, his other crew mate (Serena Aunon-Chancellor) will be after Gerst.
His seat is right at the back of the room, nearest the door he came in from, after suit fitting.
At great distance is the glass separator, in the middle of the room, with crew family & friends and press on the other side.
The crew can only talk verbally when they are closest to the glass screen (with the mic) after completing their leak check.
So during this time, if they want to communicate with their family & friends, they are using hand signs or other forms of non-verbal communication.
(Whilst waiting, Gerst also talks to his back up crew, and towards his family (and bearded friend) he gestures to his legs and also talks to support staff about his ventilator box)
- and this is the point where Alexander makes the hand signal.
(which is now off-screen)
So, it might really just be a gesture known only between himself and his family (and the bald chap with the beard who seems to be a close friend).
And.. it might be a reference pertaining to the surprise appearance to play along with Kraftwerk later on during the mission..
Video is here: preflightactivities
--> original answer (based on 2 news articles) follows:
ok, not with any real source other than this and a number of other bits of speculation:
which translated to:
which was referring to 2018 FIFA World Cup football match between his home country Germany and Mexico, which he then watched from the ISS.
...and then we get to my speculation:
He hails from Künzelsau, Baden-Württember in Germany, and in honor of his achievements and what he means for his home town he was made an Honorary Citizen of the city and an observatory was built in his name.
He received a Welcome Home party there after his second mission.
(Zurück in seiner Heimatstadt Künzelsau plant er schon seine nächste Mission: in drei Tagen zum Mond. Welcome-back-Party am 18. Mai 2019 mit Alexander Gerst in Künzelsau.)
He studied at Karlsruhe Institute of Technology, and unfurled and displayed a special KIT banner whilst on the ISS.
German electronic music group Kraftwerk, of which Alexander is a fan, had a lo-fi K as a logo early in their career, and Gerst made a surprise appearance from the ISS when Kraftwerk performed at a festival in Stuttgart in 2018.
After both Gerst and founding member Ralf Hutter addressed the crowd they then played a special duet version of the track 'Spacelab', for which Alexander had a tablet computer configured with virtual synthesizers on board.
I guess you can see where I went with this...
His other half's name starts with L though.. :/
(and doesn't seem to be a krautrock or gang sign either..)