This isn't fraud, nor is it griefing.

It *is* kind of a rude thing to do. Let's break it down a bit, though.

Typically, at least in my personal experience, fraud in MMOs is having an agreement for a trade and one party either delivering a dummy item or otherwise not holding to their end of a bargain made. For example, lets say two items have the same icon. One of these two items is incredibly rare and in high demand, the other is unremarkable and incredibly common. You agree to purchase the rare item directly from Player A, but when the trade occurs, Player A swaps in the common item. Player B in that scenario is the victim of fraud.

Unless the party finder specifically said "I, the GNB, have my 635 weapon and if it drops it will be free roll", no one in the party was defrauded.

A player deciding to trash (which is lame) an item to roll need on one that dropped isn't griefing anybody. This is something I'll do in dungeons, though I desynth the item rather than trash it. Would I do it with on-content BiS weapons? No. That's why it's rude for the player to have done.

Being able to roll need over being able to roll greed is just how the game goes. If someone else in the party wanted the gunblade, they should gear and play GNB.