A very interesting discussion.

In terms of possible implications in-universe, the Federation
does have an undeniably expansionist mindset, the Humans in particular. Not a violent one, but the case can certainly be made that it's often a somewhat
thoughtless one, perhaps all the more so because they have no hostile intention and so can't perceive themselves in terms of an overbearing force. A small outpost of Humans, from a Human perspective, looks vulnerable and harmless - a few dozen farmers, and those alien brutes slaughtered them! To a people like the Gorn, that's potentially the vanguard of a great swarm of aliens; depending on where you're standing, the same people or the same actions can read very differently. The Federation, I think, has both very good intentions (for the most part) and a very aggressively expansionist approach to interstellar exploration, and that combination gives them, perhaps, a bit of a blind spot.
A hundred years later and Cestus is a Human world revitalizing old Human sports traditions? Well, why not, the Gorn gave it to us! Everything's fine!
And back on Gornar factions of the Gorn grumble and whisper. Where do we make a stand?
In
Arena, Kirk and co clearly understood the Federation's mistake, and acknowledge the Gorn as an unintentionally wronged party, and it would seem, then, that they logically grasp the implications of the incident - that in this region, at least (remembering that the settlement of Cestus III is part of the general Taurus Reach colonization push, conducted with unusual haste), the Federation and its member states aren't taking the time to investigate the surrounding area, and are making claims without more than a cursory examination of the region. The problem, I think, is that the Federation is a big, diverse and often fast-paced society, whereas players like the Gorn are smaller and slower. The Federation bounces back at a speed that might not be healthy. The Federation as a whole has got a lot of momentum in "boldly going", after all, and it's certainly noteworthy - and possibly rather problematic - that while the Federation wouldn't knowingly invade Gorn territory, it
will eagerly take advantage of the ceding of Cestus - and if some of Bacco's later comments are considered, all but put the Gorn out of mind. Culturally speaking, the idea of Cestus as a Gorn world was quite quickly done away with, to the implied extent that the Gorn weren't even a part of the new Cestian worldview, other than as non-talkative neighbours. So a certain degree of "shoving aside" and quasi-colonialist mentality is still very much implied.
To Humans, the dispute is settled - "jolly good of the Gorn to be gracious and let us have the planet, I guess they know they were wrong to kill those people, I'm glad they're reasonable" - but to a conservative, ancient, cautious cultural mindset like the Hegemony's, the ease and speed with which the Humans claimed Cestus even after it was ceded might well be a very sore spot. "Mammalian hordes, encroaching thoughtlessly on the Gorn domain! They wrong us, and then when we are gracious and make concessions, they brush our generosity aside and go right back to expanding and multiplying along our border!". Resentment might well be justified - and we'll see that some elements of the Warrior Caste
never accept the ceding of Cestus and surrounding systems, to the point that they'll eventually overthrow the Political Caste (or whatever it's called) to take action. Again, this unfolds on a
Gorn timescale - a hundred years for that sentiment to build to boiling point, a timeframe in which the very idea that Cestus III was ever a holding of the Gorn seems to have been removed from the Federation civilian's consciousness.
I think a case can very easily be made that the Federation is rather insensitive at times, if only as a consequence of its size and the pace of its progression. By the time the insult to the Gorn has faded, here comes another wave of chattering Humans who feel that they owe the Gorn nothing (and from their perspective, it's entirely reasonable that they don't), and impose all over again.
Indeed, we'll see that this is a reasonably common complaint among the powers given to slow growth or consolidation over rapid expansion: certain Romulan governments, the Tzenkethi, the Tholians, most definitely the Gorn - all will call the Federation out on its expansionism and its hunger for new territory and new member worlds. I think a very strong case can be made that the novels do make an effort to detail what the Federation's gung-ho expansion across the galaxy looks like to some of its neighbours, and why it can cause problems.
It will be a long time until this project gets there, but how many in the Federation
really understand the Typhon Pact's paranoia?
(On the subject of Gorn claims, I've decided that my
STO Gorn commander is secretly supporting the cause of renewed Gorn independence (in the
STO extrapolation, the Gorn have become a part of the Klingon Empire). I take care never to accept duty assignments that involve suppressing Gorn revolts, for instance

).