class MyTable {
    Long revisionGroup
    Integer revisionNum = 1

    ...

    static constraints = {
        revisionGroup unique: 'revisionNum'
    }
    static mapping = {
        revisionGroup generator: 'auto'
    }
}

//Save new data.
MyTable myTable1 = new MyTable().save()
//Do "soft update" of data by saving new revision.
MyTable myTable2 = new MyTable(revisionGroup: myTable1.revisionGroup, revisionNum: myTable1.revisionNum + 1).save()
//Save new data.
MyTable myTable3 = new MyTable().save()

myTable1: id = 1, revisionGroup = 1, revisionNum = 1
myTable2: id = 2, revisionGroup = 1, revisionNum = 2
myTable3: id = 3, revisionGroup = 2, revisionNum = 1