A gerund is a verb ending in -ing and used as a noun.
Sports are often referred to in gerund form.
Examples: I like playing baseball.
Doing the audit is duty, not pleasure.
A gerund phrase is a noun phrase made up of a gerund plus any complements of the gerund plus any modifiers of either the complement or the gerund.
In the first example, playing baseball is the gerund phrase (the gerund, playing plus its direct object baseball.)
In the second example, the gerund phrase is doing the audit (the gerund doing, its direct object audit, and the, which modifies the direct object).