I didn’t comment the letter, I commented your idea that he lived through the Terror and thus the letter. No, that wasn’t the case.
And once again: the terror the society experienced that was vivid to Gagarin was from the Germans and not from Stalin (although indeed it experienced both).
I don’t want to speak to the tone of the letter, you may interpret it as designed to do something and you may not. But to say that it is somehow influenced by the terror that happened before his time and not to someone in his family, the terror about which for all we know he knew nothing, and in society that had a major uniting experience since, is to put your thoughts in his head.
