I know it is old post, but my comment can make someone happy. In order to specify watermark in bytes values (gb or mb) you have to add cluster.routing.allocation.disk.watermark.flood_stage to your elasticsearch settings file - elasticsearch.yml. Complete example:
cluster.routing.allocation.disk.threshold_enabled: true
cluster.routing.allocation.disk.watermark.flood_stage: 200mb
cluster.routing.allocation.disk.watermark.low: 500mb
cluster.routing.allocation.disk.watermark.high: 300mb
Note: without specifying cluster.routing.allocation.disk.watermark.flood_stage it will not work with bytes value (gb or mb)