GP lists are not fully supported by gp2c. A partial support is available with the list type. You must tell gp2c that a variable will contain a list by using L:list inside a declaration, where L is the name of the variable as explained in Section 3.
Currently, affecting to a list element (L[2]=x) will not work and lists will not be freed unless you explicitly use listkill.
Note: The PARI user's manual states that list are useless in library mode.