gcache.page_sizes and gcache.size are set at the same time. When will gcache.pagebe used?
The gcache is a ringbuffer and the size of it is set with gcache.size and write-sets are appended to this file and the file can be reused.

If the transaction write-set is large enough not to fit in the the gcache ringbuffer then an physical disk file, a page, is allocated to cache the write-sets. Note that there are two types of pages:


A page with a standard size as defined by gcache.page_size (default=128M as above).
A page with non-standard page size is created if the transaction is large enough not to fit into a standard page. With a gcache.page_size of 1M and transaction write-set of 1.5M will result in a separate page (in turn on-disk file) with a size of 1.5M.