According to this random website, the letter was opened in 1968 on the actual event of Gagarin's death and declassified in 2011 to celebrate the 50th anniversary of his flight.
The translation of the Gagarin’s letter in this article, which explains that it’s “translated by Yves Gauthier … author of a magnificent biography in French on Youri Gagarine” reads better than the translation in TFA, in my opinion
Sceptic and cynic in me thinks that this is a part of the russian whitewashing company.
I want to be wrong.
Please keep nationalistic flamewar off HN. It's not what this site is for, and destroys what it is for.
(I appreciate that wasn't your intent, but we have to go by likely effects.)