I'm gonna repost this in case someone can answer me now that America is awake. :3
I highly recommend Evade Extender on HBG. I was able to use it with Rath Soul because I found a nice charm, but it's tough in high rank. I used that set all the way to G rank though.
Evade Extender is important not only to keep you out of harm's way, but to keep you in the proper range. For normal 2/3 that generally means as close as you can safely get, but pierce shots need a little distance from the monster and it can be tough to get the right positioning without evade extender.
Rath Soul comes with Normal Up which works great for the Rathian gun. Critical eye and earplugs are good for almost everybody. The Najarala gun is even better at firing Normal ammo and truly shines once you can take the limiter off (when upgraded to Rare 6).
For pierce gunning, Dahren Mohran's set (specifically the alternate one based on Jhen armor) is great. Pierce up and recoil down work perfectly with the Kushala Daora HBG. That's a bit further into high rank and it's up to you if you want to farm that stuff before moving on. Earlier on Yian Garuga's set is almost exactly like Rath Soul just with Pierce Up instead, and the Nerscylla HBG is okay at firing pierce. Garuga is a real fucker to gun if you don't have evade extender though.
It's one option, though Rath Soul's natural Health -12 will be even harder to get rid of if you want to bring in Evade Extender (and what HBG user doesn't want Evade Extender?) unless you've got a really good charm. But there aren't a lot of great high rank gunner sets in general, so it's hard to recommend much better.
Oh yeah I forgot about that. I actually had some issues there, thankfully the Najarala gun has lots of slots.
For a while I had to deal with the negative health - I just brought Mega Nutrients along on quests, lol.