Recently started to do merges for Qt Project again, there are some mergers, then perhaps there are some duplicate work. Better to find them before creating new merges.
jkt in #gerrit channel on irc.freenode.net answered my question, “message” is for it.
Documentation in Gerrit Doc: Gerrit Code Review – Searching Changes.
message:'MESSAGE'
Changes that match 'MESSAGE' arbitrary string in the commit message body.
For me, I could just try “message:”Merge remote-tracking branch””.