Selectivity function argument: Const -> Array

I want to write a selectivity function for GIST indexes.

The select condition is somefield && array[1,2,3],

But when I'm inside selectivity function I get args: T_Var, T_Const.

So function needs to get the actual array contents from T_Const .

How to do it ?

P.S

T_Const is like

$18 = {xpr = {type = T_Const}, consttype = 1007, constlen = -1, constvalue =
138884428, constisnull = 0 '\0',

constbyval = 0 '\0'}

Thank you!