Is it okay to say "outside something" without "of" like "outside of something"?
Yes! It is perfectly acceptable to omit "of" in sentences like these and still be grammatically correct. However, this can't be done in every situation. It's a case-by-case situation. Here are sentences where you can remove "of." Ex: I'm outside the gate. Ex: He lives just outside of New York City. Here are sentences where you can't remove "of." Ex: We are outside of the Appalachian mountains. Ex: She is outside of the lake.