It costs nothing.
No card is asked for anywhere in GSP. There is no trial that ends,
no tier you are on, and nothing switches off later.
Then what does it cost to run?
Almost nothing, and that is the honest reason this is free. Your computer does
the work — your machine, your electricity, your disk. GSP is a program that
starts a server on it, and a program costs nothing to copy.
One part is not free. When your connection cannot hand out an address of its
own — which is most homes now, because providers share one between many
households — your friends' traffic goes through a relay we rent. That is a
real machine with a real bill every month, paid by one person.
An evening for five people moves a few hundred megabytes. That is cheap, and
it is not nothing, and a product that pretends otherwise ends with a relay
that quietly stops working one day.