The gerund is a verbal noun, "doing", in -ndi, -ndo, -ndum; it has no nominative (the infinitive does that job) and does not agree with any noun.
A gerund can take an object (honorem … vindicandi), but Latin more often makes the object the noun and the gerundive agree with it (honoris vindicandi); see the gerundive.
To tell it from the gerundive: the gerund is always neuter singular and stands alone; the gerundive agrees with a noun.