I'm not as positive about this story as most of the commentators above are. I mean, there's a lot to love here. Especially, I really liked the way the story played with the narrator's voice, making us emphasize with Gahka *because* he/it is unreliable. And I really like the themes of morality and perspective. But there are a few issues with the story that undercut these themes and made the less successful than they should have been, I think.
1 - The first, less important issue is how contrived the whole thing is. There's a whole planet here, and the two visits from Earth happened to be within a few meter of each other? I could forgive the coincidence if it was the only issue, but in conjunction with the others below it glared at me.
2 - The second, more important problem I had with the story is the whole shooting of Gahka's alien friend/ward, and the discussion afterwards. That was just overkill, in my opinion. They moved from being selfishly destructive to being cartoon villains. Gahka's actions in the end could have been as easily motivated by the realization that the terraforming meant genocide of his alien protectees. There was no need for anything as crude or unsubtle as that. (Actually, I was also unclear on why they were draining the swamp. Were they planning on terraforming the planet one inch at a time?)
Now, one thing worth considering is that in all likelihood, Gahka's actions didn't save the planet, as unless Earth is terraforming so many planets it has a policy of abandoning them the moment anything happens to a ship, or unless they keep having their ships land at exactly the same spot, then the next ship down will be able to plant its machines uninterrupted. This is not in itself a weakness; Gahka's actions are all the more poignant for being futile, and buying his alien buddies a few more years or months is better than nothing. But that would have made the moral consequences of the story far interesting if the terraforming was the main motivation for Gahka's killing of the crew.
I'm not saying the story is bad. Far from it. Nor that the moral issues it raised aren't interesting. But I have a definite sense that the story took the easy way out by making the situation so clear-cut. Which is a shame.