Contrary to some descriptions, Schonefeld airport, to the south of the city, was not in East Berlin, but just outside it in the mainstream GDR, and thus the whole additional panoply of restrictions for the country were applied to those using it. At least the runways were parallel to Tempelhof/Tegel. There was a special regular GDR bus which left from by the Zoo station in West Berlin, direct to the airport, on which documents and tickets were substantially checked at boarding, then again at the southern zone boundary, then again on alighting at the airport. It appeared to operate mostly empty.
The comments about Britannia 737s operating into Gatow are interesting, this was part of the longstanding contract they had for the UK military bases in West Germany, into other points as well like Wildenrath. Onetime university colleague became a translator for the MOD in Whitehall, and regularly travelled to Germany; if with the Minister it was BA from Heathrow, but sometimes Ministry parsimony prevailed and he was on a Britannia charter from Luton, just him in his meeting suit for a discussion on arrival, among the squaddies, squealing babies in bassinets hung from the overheads which swung in turbulence, etc. His descriptions of such flights entertained several of our later get-togethers.