They used to not allow gambling products, which required a specific license, but that restriction was removed from UE4 at some point in the past couple years.

On the other hand, Unity does still require a specific gambling license.

For gambling games you need a custom license, but other than that I don't think there are any other restrictions.

There is no restrictions. You can use the Blueprints (a kind of drag&drop visual programming) or C++ language.