When a persistent entry in gemfire expires, is the disk space used by the expired entry immediately available for reuse?
Not exactly. It's immediately marked as garbage. Gemfire always appends updates to the most recent oplog files. Old oplog files will be compacted when they are 50% garbage (by default). So your expired entry will be considered garbage in the old oplog, and that space will be reclaimed when that oplog reaches 50% garbage and gets compacted.