system
Given the following post, respond as if you're commenting on Reddit:
human
[WH40K] What if the Imperial Guard managed to defeat a Grey Knight purge

So the Grey Knight dickheads will kill anybody who witnesses them doing their thing, Space Marines and guardsmen included.

So what if the guard, being the badass normals they are, manage to resist? The Knights try and do their purge but the guardsmen somehow manage to band together and wipe out the attacking Grey Knights.

Would a battlefleet bombard the dicks out of them? Would they be considered heretical?
gpt
The Grey Knights/Inquisition would immediately assume that the guardsmen had become corrupted (after all, how else could mere mortals withstand the Grey Knights?) and devote every resource they had to their eradication.  Depending on the value of the planet itself and the extent of the resistance, Exterminatus might even be used as a measure of last resort.

With respect to Space Marines, it is noteworthy that the Space Wolves involved in the First Battle of Armageddon were not themselves marked for execution; rather, they vehemently opposed the Grey Knights' mandate to execute the guardsmen.  In a hypothetical escalation scenario, either involving Space Marines in direct support of the guardsmen or previous and notable opposition from said Space Marines to the erasure of witnesses to the Grey Knight Purge, the Grey Knights might also declare that the company or chapter involved had become heretical (or at the very least renegade).  With the living guardsmen still at issue (rather than dead and therefore no longer a point of contention for the Space Marines), expanded hostilities could lead to a much more serious rupture of Imperial unity.

The same could well be said for the higher guard commanders or the local Imperial government, depending on the degree to which the Grey Knights or Inquisition liased with these.  If the Inquisitors opt for their usual skullduggery (i.e. saying little to nothing to the officials overseeing the guardsmen they will eventually attempt to purge), mistakes over their intentions could draw higher elements of the hierarchy into the matter.  Depending on how quickly the appropriate leaders initiate discussions and come to an arrangement with each other, this might lead to a far broader labeling of heresy and devolution of the situation into open and disruptive warfare.

TL:DR Scrappy guardsmen fighting off the Grey Knights sounds inspiring, but it probably means death for far more people.