SNAP is developed in Java.If you want to know more about the internals, because you think about extending SNAP on your own, I suggest to have a look at the Developer Guide in our Wiki.
If you simply want to call the executables gpt or pconvert from the command line, then you don't even need Python. Just call them from a batch or shell script. If you want to call the API of SNAP from Python, then you need snappy (which can be installed during SNAP installation or by calling or by calling snappy-conf) and of course you need a Python installation.
I would suggest that you have a look at this post. It is good collection of entry points in our documentation.