Of is a preposition, so you're right.
There is used to for past habits (I used to drink a lot of coffee), which is not followed by a gerund, and be/get used to (= accustomed), which is (I'm not used to working with this new team). I think it's a matter of learning where you have to + -ing. If you come across an example, then try to remember it.
Student or Learner