Franck,
By using Google, I noticed a TOCH thread started by Dénes, who asked for which training units did use Gifhorn airfield by April 1945:
http://forum.12oclockhigh.net/showthread.php?t=20782
So, thanks to ChristianK, we know that II./JG 105, II./JG 107 and I./JG 110 ended the war there. The airfield thus existed and was operational; this doesn't solve thought the unit from which "Rote 20" or "Gelbe 20" did come from, as none of the mentioned units did have a III Gruppe...
Cheers
Marc