mpz_set_str (mpz_t rop, const char *str, int base)
These functions assign new values to already initialized integers.
Where are the Initialization Functions?  They should probably.
mpz_init (Gx)  mpz_init (Gy)  mpz_init (p)  mpz_init (n) 

https://gmplib.org/manual/Assigning-Integers
https://gmplib.org/manual/Initializing-Integers