I believe something like:
log4j2.appender.out.strategy.type=DefaultRolloverStrategy
log4j2.appender.out.strategy.max = 7
will keep only 7 previous versions, and to compress using gzip simply change the filePattern option to read:
log4j2.appender.out.filePattern = ${openhab.logdir}/openhab.log.%i.gz