A dropper is a player who leaves a match just before losing it, and refuses to complete the match.
Fortunately, droppers are not a significant problem on GridGammon (unlike FIBS, where droppers are common) because GridGammon removes the incentive for someone to drop with GridGammon's automatic match resolution (AMR) feature.
AMR only applies to rated matches. After ten days of inactivity, unfinished rated matches are automatically resolved by a robot evaluator. Unrated matches are simply deleted 31 after days after the last play.
If you have an unfinished rated match, you will receive a GridGammon mail message a few days before the ten day limit alerting you that the match will be resolved by AMR You will receive another message when it has been resolved for you by AMR.
More information about AMR--including technical details about how AMR resolves matches and awards rating points--is available on the GridGammon web site.
You can also gag or blind a known dropper. If someone is on your gag or blind list, any invitations from that player will be automatically declined.