As an experienced duelyst,
You need to show a pic of the deal and waged items/tc
You must have a replay/pic of you winning
If he said "gtg" before the next fight, it probably means that you'll end up FINISHING the duel. If he sends the TC/Items to an alt or something like that, he gets banned for dueling without the waged tc or items
If he said "gtg" in the middle of a fight, it's either considered a loss or you'll have to FINISH the duel. Depends on the decision of the moderators in the Scam Reports board. The most active moderator in the Scam Reports board is phail, I suggest shooting him a PM.