There are two pretty consistent rules for transitive/intransitive verb pairs, one of which should help you here.
- When one verb rhymes with ある and the other verb rhymes with える, the verb rhyming with ある is intransitive and the verb rhyming with える is transitive.
- When one verb rhymes with える and the other verb ends in す, the verb rhyming with える is intransitive and the verb ending in す is transitive.
With your specific example, rule 1 applies. So 集まる is intransitive and 集める is transitive.