If it is damning, and suggests Mr Johnson broke Covid rules by going to a “bring-your-own-booze” party on May 20, 2020, when the UK was in lockdown, he could be left clinging to the keys of No10 by his fingertips.
Pressure would inevitably pile up on the Metropolitan Police to launch a formal investigation which would leave Mr Johnson in an extremely perilous political position.
He has defied political gravity for years and is yet to come thumping to the ground from the Westminster high wire, though the wire is looking more and more frayed, some would say threadbare.
Ms Gray could report as early as next Tuesday and her report is certainly expected within the next few weeks.
However, the rules and laws on Covid restrictions do not always appear clear cut, as was evident from former No10 senior adviser Dominic Cummings’ trip to Barnard Castle.
He was not found to have breached them, even if his trip to Durham and the picturesque surrounding area was in the eyes of most people at least glaringly not in the spirit of the regulations.
Scotland Yard is also wary, or at least appears wary, of getting dragged into the political arena and there is a case, possibly not very strong, that there are more important alleged offences to pursue in the capital than past parties at No10, no matter how flagrant a breach of the rules they might have been.
Ms Gray spent six years as director general of propriety and ethics in the Cabinet Office, she also ran a pub in Northern Ireland and was once described by former Cabinet minister Sir Oliver Letwin as the person who “runs” Britain.
She has led a series of inquiries including into the “plebgate” row, which led to the resignation of then Chief Whip Andrew Mitchell despite his strong denials of having used the “pleb” word.
She is expected to identify, though not necessarily publicly, who was at all the parties, as well as when they took place, how they were organised and what the Covid rules were at the time.
But it is far from clear whether she will make any decisive judgement on whether any laws were broken, a role normally left to the police, Crown Prosecution Service and courts, rather than the Civil Service.
Even if she does suggest laws or guidance were broken, she will almost certainly not propose what disciplinary action should follow, certainly for any ministers.
Cabinet Secretary Simon Case might decide on disciplinary steps against civil servants found to have breached the rules but, perhaps ironically, the Prime Minister’s reprimand, if any, may be left to…the Prime Minister.
He might also refer it to his watchdog Lord Geidt, who cleared him of breaching the ministerial code on the Downing Street flat luxury revamp.
Public outrage, if her report is very critical, could lead to more MPs moving against Mr Johnson.
But while the Conservatives can be ruthless in getting rid of their leaders once they regard them as a liability, it is still a step many would take very reluctantly.
Fifty-four MPs would need to write letters of no confidence in the PM to Sir Graham Brady, chairman of the 1922 Committee of Backbench Tory MPs, to trigger a vote on Mr Johnson’s future.
Many threaten to do so and many are very angry, particularly among the 2019 intake, at what they regard as self-inflicted wounds on the Government.
But only Sir Graham knows how many letters have been put in and it is often less than speculated.
So, Mr Johnson may get over the “hill” facing him.
The “mountain”, though, is the May local elections.
Before then, Covid is likely to be firmly in retreat as the country comes out of the winter and into spring when the virus will find it harder to spread.
However, by then millions of families could also be under financial pressure from the looming cost-of-living crisis of high inflation, soaring energy bills and rising taxes.
So, the backdrop to the town hall polls could be politically and economically turbulent.
The elections will also give MPs a firm insight into their prospects of holding onto their seats at the next General Election.
Many Tory MPs in former Labour “Red Wall” constituencies have seen the majorities which got them into Parliament in 2019 evaporate in recent months.
Double-digit leads, for some, are now double-digit deficits which they have to recover even to get back to level-pegging.
So, May looks like the most dangerous crunch point for Mr Johnson.
By then, his colleagues will have weighed up whether he is a benefit or a drag-anchor on the Tory party’s prospect and will seek to cut him adrift, or not, accordingly.
If a confidence vote is called and if he won the backing of more than half of Tory MPs (181) he would remain Conservative leader, and Prime Minister, and could not be challenged for a year, under the current rules.
But if he lost, he would have to stand aside for another Tory leadership race.
Chancellor Rishi Sunak and Foreign Secretary Liz Truss are already emerging as frontrunners.
But Education Secretary Nadhim Zahawi is also being tipped as a strong contender, having as vaccines minister played a key role in the battle against Covid…for him, the jab might just get him the job.