I would say thank you, and add very kindly you need him to stay out of your life. Then leave it at that. If it's truly behind you, you don't have to say anything else; otherwise, all you're doing is baiting him.

Mark it return to sender and send it back.

As much as you want to respond by rising to the bait, I think it's probably just going to open up the past. Do you want to go around and around with him again? If you really want to get under his skin, I'd suggest that you just let it go. If it is a provocation rather than a gift, I don't see any reason to thank him. He's already told you who he is. Believe him.

After seeing this "gift"...yeah....not a "gift" but a dig at me trying to get a rise. The old saw holds true still...the leopard hasn't changed his spots. Silence is golden.

I'm beginning to see things in terms of "trolls". Not the sort that live under bridges in fairy tales, but the sort that troll for attention. They set the bait out there on the end of a string, hoping to get a nibble. When they do, they try and reel you in. Could be that this is sort of like that. The gift could be the bait. If you contact him, he'll try and reel you in in some way. Do want to be reeled in?

Don't respond.

I would not respond at all. Pretend it didn't happen. When dealing with the kind of person you describe, silence speaks louder than words ever will. I just went through a similar situation so I'm basing this on my own experience & regret ? Good luck!

Like most dudes, for me, when it's done, it's done, especially when it happens by way of mistreatment. I would not respond, or feel any obligation to. You mention gaslighting as a favored tactic. This does not mean he's got borderline personality disorder but he may well, in which case, having had a partner like that long ago, no, I would not re-establish communication as it's crazy-making and no good can possibly come of it.

Just thank him and then stop all further interactions by ignoring anything further.

Say the past,is the past,do not believe of ever being a couple again,you had your chances and blew it......

Fuck him. Wait - not literally; more in the "your dead to me" mold.

Basically, I wouldn't respond at all. He doesn't deserve it.

Based on your info provided, X sounds like a typical manipulator. Manipulators have to be communicated with in order to manipulate you. Regardless of how strong your will is, with communication X can wear down your resolve over time. My advice is do not respond to, or open communication with X at all costs. Do not keep any gifts or read any more messages that X sends you. You don't owe X anything! X choosing to send you gifts of his own volition, does not create a debt that you must repay.

My wife and I are going through this with a good friend of hers that is "too nice" to say no. She is getting better at it though because after all these years, she is finally reaching the emotional point that you described in your title. The correct phrasing of which, as I heard it long ago is; the opposite of love is not hate, but indifference. Apathy works too. Good luck!

Would you bring rotten garbage into your house, put it on your kitchen table, and call it "dinner"???

I wouldn't take him back if he were the last man on Earth. I wouldn't even fuck him if he were the last man on Earth.
When anyone treats you that badly, there is NO reasonable expectation that they will ever change, no matter what they say.

You are going to make your own choices, but since you asked, I strongly encourage you to tell him to fuck right off. Change your number, move if you have to, but do not allow him back into your life.
Don't even thank him for the gift. He owes you a helluva lot more than some paltry trinket, no matter what it is.
Move forward with your life as though he's dead. Can't communicate with the dead. They don't exist. Neither should he.

A simple "leave me alone, dickwad" couldn't hurt. Then silence. YMMV

I think you have to really evaluate whether you've healed. This may be an excellent opportunity to get any lingering resentment out. There are mature ways to say "Thanks, I'm glad you've been enlightened of your cruel behavior, but you were a dick to me and we should both carry on with our separate paths."

You just have to consider how much of that trauma you want to continue to carry. I would definitely lean on those former partners, if possible. Overall, just keep your mental health in mind and resist an unnecessary rabbit hole. Good luck!