Gerund: used as a noun to denote the act or process of justifying.
His constant justifying of his mistakes grew tiresome.
Его постоянные оправдания своих ошибок надоели.
justifyinggerund / present participle Present participle used attributively.
She offered a justifying remark to explain her absence.
Она произнесла оправдывающее замечание, чтобы объяснить своё отсутствие.
justifygerund / present participle Used attributively in phrases like "justifying explanation"