there are two things to consider here:
1. gusta or gustan: we use gusta when what is liked (in this case "este documental") is a singular thing, so "A vosotros no os gusta este documental."
2. You can easily drop the first part saying "a vosotros" and just say "No os gusta este documental", only using "os". But we are using also "a vosotros" to emphasise who does the liking which is "you plural", in which case you add "a vosotros". Bear in mind that we can state in the sentence who likes whatever by using "me, te, le, nos, os, les" and using "a mí, a ti, a él, a ella, a nosotros, a vosotros, a ellos"; both of these have the same function in the sentence. Sometimes we use them both to emphasise. Here is a lesson on "repetition of indirect object pronouns with verbs like gustar". Have a look here.
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