Yes, you only need one exotic or raid piece to get to 29 if you have a full legendary set.
Gjallar is light years better than Hezen's if you ask me, and not just because of wolf pack rockets. It tracks pretty darn well, so you'll nearly always hit your target unless you haphazardly fire it off without at least pointing at your target.
My gjallarhorn does ~12,500dmg to Aetheon give or take on the initial blast, then all the wolf pack rounds do around 1000 to 1500. I can't figure out how many of those small rockets come out, but judging on damage spam popping up I'm guessing around 8 to 10. That means one rocket from my gjallar does around 20,000 to 25,000dmg to Aetheon, or the equivalent of two crits from Icebreaker, except that I don't have to line up and get a crit. I can just turn, look, and shoot off two rounds, then reload while jumping/dodging fire.
I seriously cannot overstate how powerful the Gjallar is in PvE. Hezen's veneance cluster bombs just drop straight down, meaning they sometimes don't even damage the enemy you initially shot at if it moves (think minotaur teleports), and it doesn't track meaning you may miss sometimes (or often depending on skill).
If Xur sells gjallar again, everyone, EVERYONE better buy at least one. Maybe two just to be safe.
I had the coins... I had the coins...