do any gamma rays reach the surface of the earth?

Most won't. Very rarely, some might, but it is not possible to detect them as really really really too rare.

And if so isn't space supposed to be empty?

In average, space is empty. But there is stuff in it here and there.

what's stopping gamma rays from reaching the atmosphere ?

Gamma rays can reach the atmosphere. The atmosphere is actually the stuff protecting us from them :)