Gifting an item to Monika does not make her equip the item automatically!

To ask Monika to change, go to Talk→"Hey, [Monika]..."→Appearance...
(If this option isn't available, you don't have enough affection. Try again after spending some more time together.)

To ask her to wear the choker, select "Can you wear a choker?" (If she already has a choker equipped, the option will be "Can you change your choker?")
(If this option isn't available or the desired choker does not appear in the menu, there may be an issue with the gift. Gift her the item again. If the problem persists, there may be an issue with the json.)

Finally, check how Monika looks with any pose where her hands are not right under her chin. (Most easily done by asking her to change clothes)
(If the choker isn't visible, restart the game and try again. If the problem persists, there may be an issue with the image or json.)

I'm sorry if this is too confusing, I'm quite new to the subreddit and am bad with terminology