In Italian it follows the -are,-ere,-ire endings:

mangiare/ mangiando

credere/credendo

dormire/dormendo

avere/avendo

essere/essendo

It does not matter if the verbs are either irregular or regular. It is the exact equivalent that English one. The only difference is that gerund in English can also serve as an infinitive verb at the beginning of a sentece, in Italian does not.

Of course, we can have past and present gerund, but the ing-form does not change.