Show boss Dominic Treadwell-Collins said her on-screen son, Adam Woodyatt, was so shocked by the news that he burst into tears.
Viewers saw Gillian’s character, Kathy Beale, return on Thursday, almost 10 years after she was said to have died in a car crash off-screen.
Gillian signed up for her return in November and was sworn to secrecy and only told her partner David Fairbairn.
She said: “It’s been awful and the only person that knew was my other half Dave. I was trying to say to the children the other day, ‘try and watch EastEnders on Thursday’, I said to them, ‘because it’s live and Adam is going to do something really silly so everybody starts laughing’ and I thought that will get them watching so they can see it then and go ‘ohh’, but my daughter said ‘I’m working until nine Mum’ and my son said ‘I’m playing football over the rec’, so I went ‘oh fine, thanks very much’.”
Dominic told Adam, who has played Ian Beale since the BBC One soap started, just before Christmas.
He said: “I had this meeting with Adam and he came to my office and I said ‘look, we’re bringing someone back’ and he looked at me and he went ‘not my mum’ … he broke and he started sobbing and sobbing and sobbing.
“He cried for two minutes, he cried from his gut, and he looked at me and said, ‘Really? really?’ and he said, ‘That’s all I ever wanted, all I ever wanted is for my Mum to come back’.”
The producer, who said the character would return in a “credible” way, said he had disagreed with the original decision to kill her off.
He said: “I’ve always wanted Kathy back. Kathy is one of my favourite ever characters.
“When I first arrived here 10 years ago I was at the story meeting when they said ‘right, and Kathy’s died off screen’ and I remember sitting there as this quiet little newbie going, that’s stupid, why would you kill her off-screen, why kill off an iconic character off-screen in a car crash.”
Even Gillian has been kept in the dark about exactly how she will be brought back, saying: “I’ve got no idea whatsoever. I hope I come back strong because I’ve been through an awful lot, death, everything.”
The plan to bring her back was even given a codeword – OOF which stands for Our Old Friend – which was dreamt up by Adam and Steve McFadden, who plays Kathy’s ex Phil Mitchell.
Show bosses filmed Gillian’s scene in an isolated set near Canary Wharf and drafted in Anna Acton, whose character Emma Summerhayes was killed off recently, as a stand-in in case any pictures leaked out.