The door should spring open, and there you have it!
Many newer Cadillacs have a capless gas tank, so don’t be surprised if there’s nothing to unscrew once you open the lid. This system is actually designed as a security measure, as it makes it much more difficult to siphon gas.
While Cadillac states drivers can use gas with an octane rating of 87 or higher, it can cause acceleration issues and low fuel economy. They recommend using premium unleaded gas rated **91 or higher, so you might want to stick with that unless the rental car company specifies otherwise.