This is a very good question.
The main purpose of the gate review meeting (also called phase gate review meeting) is to periodically (at the end of each gate) ensure that the project is running smoothly, and take corrective measures if it's not.
These meetings are attended by a group of stakeholders, the project sponsor (who usually chairs the meeting), and the project manager (also such meetings may include some team members because of their feedback, although I personally don't think it's a good idea to do this).
These are important meetings, so they can take up to a day (it is very rare for such meetings to take more than a day, and often they take half a day)
If you want, yes, you can say that gate reviews are pre-scheduled, since they are held at the end of important milestones.
Essentially, what it is discussed during the GRM (abbreviation for Gate Review Meeting) is how is the project doing so far (still adhering to the business case? project is done according to the agreed upon quality and scope?), and should we continue with the project or just cancel it. This makes the GRM one of the most critical meetings for a project manager, as his project can be easily canceled/killed. Ideally, any decision made during a GRM is by consensus.
Again, GRMs are held after finishing important milestones. For example, after gathering the requirements, when finishing the planning, when finishing the execution of the project (just before the testing), and just before closing the project. Typically, you should never have less than 3 GRMs in your project. But just make sure to keep the number of these meetings small, they're not, after all, weekly status meetings.