Enter/Exit the Gungeon by Devolver Digital is a great one. PC/Switch releases available, and is literally gun themed. Lots of charm, lots of loves by the devs, and lots of replayability. Cuphead by Studio MDHR Entertainment Inc. PC/Switch releases available, and is based around unique boss-fights, animated in 1930s cartoon style.
Related to Cuphead, I've seen a pseudo-parody game called Cockhead by Yoba Games circulating around, which plays almost identically to Cuphead, but you're a penis with a gun. Though, it seems to have been made as a follow up to Cuphead's success, and I haven't played it myself, so...do your own research for this one. Nuclear Throne by Vlambeer is another gun-based bullet hell. PC release available (it may have others). It's high-adrenaline, sooo...yeah, have fun.