Gone and not missed at all:
Colt Combat Commander, for a long time, the most expensive gun I ever bought. A total piece of junk. Never should have made it out the door. Colt "fixed it" twice, and never got it working right. Sold it to some Colt fanboi who didn't seem to care it wasn't reliable.
FTL Auto-Nine. Neat little .22LR that blew extractors.
Taurus 83, my very first gun, and the worst one I ever had. Made the Colt look good.
AMT Back-UP .45. My trigger finger would be dead just putting one mag through it. On top of the insanely stiff trigger, it would have light primer strikes. A gunsmith got it working 100% but it still had a super stiff trigger.
AMT Hardballer Longslide. Just bad enough I wanted it gone. Killed any love, and there wasn't much, that I had for 1911's.
And all the other brand new turds I bought from about 1981 until 1989. So many bad new guns, while the used ones were great.
Gone and missed a lot:
Colt Python. So pretty, but got a nice price for it, and I didn't trust it after the hand broke in it soon after I bought it. End of my Colt ownership, period.
S&W 28. With an action job and a target hammer and trigger. Just a great gun, with enough "Patina" to show it had been a cop's duty gun at one time. Showed up on Gunbroker about 6 years ago, but I was outbid on it to the point I let the other guy have it.
S&W 4506. A PD trade in that was in almost mint condition. Had all the paperwork, including a slip showing the officer and badge number it was issued to. Found him in 30 seconds online. I sold it for a nice profit, but instantly regretted it.
Browning BDA 380. First 100% reliable semiauto I ever owned. I bought one again a couple of years ago, a PD trade in, it's not nearly as pretty as the old one was, but it eats anything just like that one did.
H&K P7. Why did I sell it? I needed money and I got a lot for it. Never had a chance to replace it while the trade in ones were sanely priced, so I still don't have one.
S&W 5906, 6906, 4006TSW, and several other 3rd gen S&W's. I loved them, but I needed the cash.