Some great Tips regarding Gatling Gun :
1- Use Bone Marrow Ashes ( They're definitely Helpful as long as you keep Holding/Shooting Bullets from this Massive Machine Gun.... Or else if you stopped , apply the Bone Ashes again for the limited stream of Bullets ).
2- It's better to equip Formless Oedons for extra Quick Silver Bullets + , rather than Oedon Writhes visercal atk Bullet gains... This is because Cannon's usually go well with Oedon Writhe Runes.
3- Put a great Bloodtinge Stat level up if you want some good damage with this thing , Cannon's dont usually need that High Bloodtinge levels but this thing needs indeed.
4- I just found out a really smart Badass idea when wielding this weapon , ( First you want to Poison your Boss Enemy , this works on all Bosses like Gascoigne , Maria , Gerhman , Bloody Crow of Cainhurst , Logarious , essentially any Humanoid Bosses or NPC Hunters ). While being Poisoned by Poison Knifes , patch up your Gatling Gun with a Bone Marrow Ashe then go BBBRRRRR!!! This'll easily deal 10K-20k Damage...
5- Lastly , another alternative method for Killing mostly Huge Kin Bosses ( Like Rom , Ebrietas , Amygdala , Cleric Beast , Laurence , Loran Silverbeast etc ). This method requires you both a fast weapon like Church Pick , Threaded Cane , Saw Cleaver with the Gatling Gun ( First Buff up with Fire Paper , then throw off an Oil Urn... then wack off the Bosses for good damage , then as you about to finish up. Patch a Bone Marrow Ashe into the Gatling Gun and shoot it up untill you Win!!! ). Goodluck , these are pretty crazy hardcore methods very useful for specialy fighting Chalice Bosses & DLC Bosses with the Gatling Gun... Enjoy hope this helps