Writing a beautiful sonnet is as much an achievement as ______ a 400 page novel.

The use of "finishing" is grammatically correct because it functions as a gerund. A gerund is a verb form that functions as a noun, and it is created by adding the "-ing" suffix to the base form of a verb.

In this case, "finishing" is a gerund that serves as a noun and is used to express an action or activity. The sentence is comparing the achievement of writing a beautiful sonnet to the achievement of completing (or finishing) a 400-page novel. The gerund form "finishing" helps to convey the idea of the action or process of completing the novel as a substantial and noteworthy accomplishment.