There is no punishment for atheism or blasphemy. (There is a Puranic theory that claims that one can get moksha faster by being an enemy of God.)
Gita acknowledges the right of a person to reject its teachings.
‘..Reflecting on this entire teaching, do as you think fit.’
However, Gita has very low opinion about atheists.
According to them nothing is ultimately real in this world. It is
Godless and without any moral basis. Being born of sex union, what
else but lust can be said to be its cause?
What happens to atheists?
The only way to end of sorrow is to know God.When men shall roll up
space as if it were a piece of leather, then will there be an end of
sorrow, apart from knowing God.
Svetasvatara Upanishad VI.20
Atheists will remain tied to the endless birth, death, birth ... cycle of Samsara.