Practice manager Karen Evans falsified patient records to divert hundreds of thousands of pounds in funding for medication, staff wages and medical supplies over a 15-month period to two bank accounts she controlled.
Ms Evans used most of the money to login to online betting apps - on one occasion winning £120,000 but losing it on other bets in just days.
She took a total of £582,265.65 from the worst-affected practice, Failsworth Group Practice near Manchester, in 327 illicit transactions. The first theft of £2,100 took place just a month after she was hired.
The theft was only uncovered when doctors at the practice launched an investigation to find out why they had to arrange an overdraft to pay staff - and found their business account was £31,000 overdrawn.
The practice began to receive claims for unpaid bills after Ms Evans had been dismissed, and one GP was forced to check through 12,500 patient records for falsified information - uncovering 323 cases in which records had been tampered with. Nine patients whose records had been falsely marked 'end of life care' had to be reassured.
Four of the five doctors at the surgery subsequently had to leave or take early retirement as a result of the thefts, the court heard.
Police discovered Ms Evans had also stolen £77,000 from another GP practice in nearby Hattersley by falsifying paperwork over medical supplies while working there between 2012 and 2016.
At Minshull Street Crown Court, Manchester, Ms Evans admitted two charges of fraud by abuse of position and was jailed for three years and four months.
Prosecutor Duncan Wilcock said: 'The surgery was run as a small business by five doctors that worked there. Since that time only one of those five partners remain - the other four have either taken early retirement or gone to new places as a direct result of the defendant's actions.
'Matters came to a crunch in spring 2018 at the practice when a £25,000 overdraft had to be arranged so there were sufficient funds to allow for staff to be paid.
'The defendant was then tasked with producing a financial report but it would appear in the following weeks she was absent from work and didn’t attend the meeting or produce the report. Instead she produced some emails that signified that the financial position was much healthier than it actually was.
'Because she failed to attend the partners asked the deputy to make some enquiries and review some accounts. She did that that same day and it showed the business account was £31,000 overdrawn.'
Defence lawyer Robert Smith said: 'She used her own bank accounts and was remorseful once she was caught and she made admissions to the police.'
He said Ms Evans had had 'many difficulties in life' including suffering from domestic violence and drug misuse. He said: 'She had a problem with drink and an addiction to codeine medication. As a means to escape she resorted to gambling on her mobile phone using apps.
'It is so simple to do - at one stage she won £120,000 and the gambling company awarded her a bottle of champagne but she lost the money in days. Her addiction was allowed to continue and the gambling companies do play some part in this.'
Judge Maurice Greene told Ms Evans her actions were 'calculated and cruel'. He said: 'You stole the money because of your gambling addiction but the abuse of position of trust you were in caused a serious detrimental effect on the victims of this case.
'It is quite clear that you are a lady with a gambling addiction and have accepted that. You are remorseful now and you do accept the need for treatment for gambling addiction but it is quite worrying that you could clock up well over half a million in a 15-month period. It’s so easy to gamble in this way and to lose money so quickly and that is what has happened.'