Construction of past gerund
The past gerund allows us to indicate that two actions with the same subject are taking place at the same time in the past. It is always introduced by the preposition "en" and the auxiliaries "avoir" and "être".
The basic rule for constructing the past gerund is to take the auxiliary "avoir" or "être" at present gerund and add the past participle.
En ayant révisé, c'est plus facile.
En étant venu, on se rend mieux compte du travail à faire.
Past gerund is constructed from present gerund of "avoir" and "être" with the addition of past participle.