Thanks.

The second part is finally a very good example for a reply that is not gatekeeping but actually helpful.
Would perhaps be a good idea to have that exact paragraph be put at the top of the community description.

And regarding the first part:
If stumbling across a post linking to inaccessible content in any community:
Instant reaction will always be leaving a “Paywalled” or similar comment.
That’s just basic netiquette to inform other users and give the OP a hint that he might want to do something about it.