Well, I certainly didn't fool any of my readers with that question. Everyone answered verbalizing and everyone answered correctly. A gerund is a verbal noun; basically a noun derived from the -ing form of a verb. So to turn the verb verbalize into a noun you would add -ing to make verbalizing.
The official definition of a gerund as listed on Answers.com is:
1. In Latin, a noun derived from a verb and having all case forms except the nominative.
2. In other languages, a verbal noun analogous to the Latin gerund, such as the English form ending in -ing when used as a noun, as in singing in We admired the choir's singing.