Why do they use que in this sentence and le before gustan?
Si a alguien le gustan las cobayas que me lo diga Gracias 
3 Answers
I understand it as "If someone here likes guinea pigs tell me!" I am guessing it's the start of a forum post or something along those lines to start a conversation
it's "que me lo diga" because it's short for "quiero que me lo diga" and when you say you "I want" "someone else" to do something you use the subjunctive, when the person wants a person other than themselves to do something you use the subjunctive.
E.g.
Quiero irme. I want to go.
Quero que te vayas. I want that you go. (This is subjunctive in English too, that's why you say "I want that he go" and not "I want that he goes" , so usually we say "I want him to go" because we avoid using the subjunctive in modern English)
Qfreed already explained the "le gustan" 
Could you tell us what the sentence means in English? Is diga in subjunctive mood [since it cannot be in imperative mood since the pronouns are not attached.] Is that Si (if) or Sí (yes)?
It neither says,
Yes, it is someone who likes guinea pigs that is telling it to me.
If someone likes guinea pigs, tell me.
I don't really understand the subjunctive "diga" and it is confusing the sentence's meaning for me.
Anyway, to explain the le.
Intransitive gustar sentences require an indirect object to tell who the subject is pleasing to. (who likes what). Guinea pigs are the subject . le, a alguien is the i.o. and "redundant" i.o.p. Guinea pigs are pleasing to someone or someone likes guinea pigs.
The le is necessary whether the "a alguien" is there or not (to clarify who le is.) Or you can look at it as the a alguien is the i.o. and it precedes the verb so a redundant i.o.p. is required.
me habéis ayudado muchisimo Gracias
