I could not find the code either.
There is a link in the comments of this video: https://www.youtube.com/watch?v=GYoTGbEgAok
The link is to a document on the vk.com website.
I don't know if there is something there, because I had to log in, which I did not do.
Do you have enough information to build the hardware ?
I think that we can make code that works. There are other projects like that. The transistor gives a pulse to pin 2, that is obviously an interrupt. Every time a interrupt is detected the buzzer makes sound for some time.
With careful code, the Adafruit GFX library can run on a Arduino Nano.
Geiger counter code just counts pulses, and typically reports the CPM, total counts per one minute of time. It is very simple, but you could use the code from Sparkfun's retired Geiger counter project.