Gavin isn't entirely sure what he was witnessing here, but there was no doubt from the knight's attire or his address to the fleeing people that this was a member of the Order of the Nail. A Hellknight.
Gavin hated Hellknights - it had started off as an impersonal dislike developed from hearing stories from veteran guardsmen, but that dislike rapidly soured into hatred after having a few scattered dealings with Hellknights during his brief career as a guardsmen. They were always judgmental, arrogant, and unflinching in their beliefs.

Given that these people were likely citizens attracted by Lamm's promises of societal upheaval, it was likely that the "judgment" the Hellknight referred to meant "summary execution for treason".
Gavin didn't much care for these idiots about to be executed either - they had shown up with whatever weapons they had available, so it was clear that they had wholly bought into Lamm's promises of a reckoning. Which did, indeed, make them traitors on top of being idiots. But . . . there was still a process to be followed, and the professional guardsman in Gavin was awakening after years of slumber.

And so, instead of minding his own business and leaving these people to face their inevitable punishment summarily, Gavin sighed and stepped forward.
He continued walking forward until he was just short of the group of fallen traitors, but he kept out of easy arm's reach from them and did not circle around to block the Hellknight's progress towards them.
Getting between a Hellknight and his quarry was a good way of getting yourself labeled an accessory in their mind.
He kept his rapier in hand for the moment, but kept it down at his side rather than brandishing it in a way that might be seen as threatening.

"Good evening, Ser Knight. Excellent work in apprehending these criminals. Do you mind informing me of what crimes they are accused?"

Gavin said with false cheer, uncertain if the Hellknight would actually appreciate being called such - some of them seemed touchy about such things. He then turns his attention to the people as his feet.

"If you value your lives, you will do as Ser Knight recommends, drop your weapons, and lie still. Those who surrender will be taken into custody and not harmed until the Arbiters decide what is to be done."

Gavin speaks this last bit forcefully, not for the benefit of a threat but instead to make sure that the Hellknight is reminded of the fact that he does not have the authority to summarily pass judgment on criminals. At least, unless he had indeed been granted special authority to do so, in which case a whole lot of people were going to die tonight - and some probably for littering.