How does the 300 calories work for an Active Day? Is it aggregated over the full day like steps?

No, you must burn 300 calories in a single exercise session. An exercise session must have a start and end time. The calories are therefore not aggregated over all your exercise sessions in a day or over the full day.

I see that I have 2 or more gym visits for the same day?

This is sometimes due to the gym provider. As per the Active Dayz rules, you will only be allocated one Active Day for one gym visit. The other gym visits will be ignored.

I have obtained an Active Day, but I cannot see the points allocated. What should I do? 

Multiply allocates Active Dayz™ as soon as we receive an activity that makes you eligible for an Active Day. However, Multiply normally allocates all points overnight. You should be seeing your Active Day™ the next day.