"Hissed", "went", "whizzing", and "landing" are all verbs in the quoted sentence. They are inflected forms of the verbs hiss, go, whiz and land. The quote within the quote contains the verb "forget".
The words "whizzing" and "landing" in this context would typically not be categorized as "gerunds". The exact definition of the term "gerund" (and whether it is a clearly defined concept at all) is actually disputed, but in general, a gerund is thought of as a verb with the suffix -ing that "acts like a noun" (for example, by being the head of a phrase or clause that is used as a subject, or as the object of a preposition). In "Professor Flitwick went whizzing resignedly past them, landing on top of a large cabinet", the words "whizzing" and "landing" don't have any "noun-like" function. They would be classified as -ing-participles (which are variously called "present participles", "progressive participles", or "continuous participles"; or "gerund-participles" if the gerund/participle distinction is rejected).