If the day for the pool says R for reserved does that mean by reservation only, or is that just the maximum number of day passes that can be made online? Will there be separate day passes available at the gate?

Best to call and make reservations. If not they can close the park to drive ups. Especially on weekends

If the day says R it means all of the "save the day" passes, which are really more like tickets, are gone. There are no 1st come 1st served tickets in addition to the reservations. They also do not make exceptions. If you do not already have a "save the day" pass and you try and go on a day that says R you will not be let in.