Basically, Microstream does not have a special impact on the Java Garbage Collection. Of course, Microstream requires Heap space. This is the heap required by Microstream itself and, more interesting the amount of data required to manage user data that also affects the GC.
Currently Microstream requires one registry entry with about 120 Bytes for each stored entity and another additional entry with about 80 Bytes for each loaded entity (please note that a java object may have several entities).
For a stored and/or loaded collection with 1 Mio entities this will result in roughly 200MB extra management data.
Entities that have not been stored/loaded by Microstream are not managed, they get registered during the storing/loading process.
Microstream uses week references to register loaded entities so those can be deleted by the standard java garbage collection if your application has no more references to those entities.