GrammarKit explanation?

Ok so, BNF is quite the mystery to me. I'm trying to find information about it, and every time I think I understand, it breaks completely.

I want to create a BNF for something very simple:

//Optional comment

script someName() {

}

I have the following BNF:

{
    //configuration for generated class names here

    tokens=[
        //Tokens
        KEY_SCRIPT = "script"
        KEY_RETURN = "return"

        // literals
        l_identifier="regexp:[a-zA-Z_][a-zA-Z_$0-9]*"
        l_identifier_text="regexp:\`[^`]+\`"
        l_decimal="regexp:[+-]?(([1-9]\d+)|([0-9]))\.\d+"
        l_integer="regexp:[+-]?(([1-9]\d+)|([0-9]))"
        l_string="regexp:('([^'\\]|\\.)*'|\"([^\"\\]|\\.)*\")"
        l_raw_string="regexp:.*"

        // base
        semicolon=";"
        parenthese_open="("
        parenthese_close=")"
        bracket_CurlyOpen="{"
        bracket_CurlyClose="}"
        bracket_SquareOpen="["
        bracket_SquareClose="]"
    ]
}

single_comment ::= '//' l_raw_string

script_declaration ::= KEY_SCRIPT l_identifier parenthese_open parenthese_close bracket_CurlyOpen bracket_CurlyClose

cs2 ::= (single_comment) * (script_declaration) (single_comment) * <<eof>>


however, IntelliJ says script_declaration is an unreachable rule and cs2 is an unused rule. 
The BNF flavor used in GrammarKit seems to be something different (with less capability?) than most explanations 
I can find on the internet. And even those are quite incomplete or unclear.

Can someone explain this and possibly provide me with some resources other than the GrammarKit github, as it leaves too many things unclear.