The actress, 56, revealed that she first realised something was wrong during a dog agility class with her lurcher Professor Scruff: 'I suddenly felt this sharp pain and it didn't go away. I made it home, a neighbour dialled 999 and I was whisked to hospital.'
Gillian was told by doctors that she was lucky to be alive, and diagnosed with acute pancreatitis - an illness which can be fatal. Speaking to the Sunday Mirror, she said: 'It was touch and go. It's no exaggeration to say I had a real brush with death.
'The pain was unbearable. If somebody had given me the choice of living with that pain a day longer or swallowing a pill to end it all, I'd have taken the tablet.
'No question. It was unbelievable.'
The pain was so bad that Gillian was prescribed morphine to help, which caused her to experience side effects including hallucinations. 'It makes you high as a kite,' she said. 'One night I was found at the foot of a bed giving a Shakespearean monologue.'
Acute pancreatitis is a serious illness where the pancreas becomes inflamed. Most people improve within a week and face no further problems but in some serious cases there can be complications, which can be fatal.
According to the NHS, acute pancreatitis is most often linked to either gallstones, which accounts for around half of all cases, or alcohol consumption, which accounts for about a quarter of all cases. It's more common in middle-aged and elderly people, but it can affect people of any age.
'I was so ill my parents flew down from the Highlands of Scotland to look after me. And my sister, Lois, and nephew, Stanley, were frantic,' she said.
Thankfully, Gillian - who returned to Albert Square last week - made a full recovery and is currently starring in the West End Alan Ayckbourn comedy How the Other Half Lives alongside fellow former EastEnder Tamzin Outhwaite.