Is this possible on DMM, then? Exchanging dupe Golds for 6* tickets?

There are no 6* tickets on DMM.

Only 6* medals as what the guy below you has mentioned, of which you need 300 to directly exchange for one just like what you do with life crystals, and this takes an awful lot of time to grind enough materials for those medals. That particular feature is barely a month old there too, so don't expect it to come around soon on this side.