You've clarified that you did not use potting mix, you used soil with added gypsum, and that the pot you've used is open at the bottom. That added information points to more than one possible problem.

First, if the pot is open at the bottom, and you did not dig over the soil beneath first, either the plant in the bottomless pot cannot take up sufficient water before it disappears straight out the bottom, or the water collects at the base of the rootball because the clay soil is solid and cannot drain away.

Second, the soil plus gypsum you used is not intended for use in pots, it's intended to be used in open ground, in particular where there is already a calcium shortage, or too much salt in the soil (usually in coastal regions). Gypsum can burn plant roots, depending on how much has been used, and if the soil mix wasn't sterile, it could contain some pathogens, again not intended to be contained in a pot. Whilst harmless in open ground, some pathogens present may well cause trouble in a pot.

The best way to improve clay soil, open it up and make it more free draining is to use organic material, such as composted manure, good garden compost, and so on, along with some horticultural or washed grit. Gypsum may have a temporary (up  to 2 months) effect on the clay, making it more 'open', but it is not usually applied round plant roots in sufficient quantity to have this effect because of the risk of root burn. If it is used on growing plants, it's in small quantities and usually to temporarily correct a calcium shortage.

Magnolias dislike root disturbance, but in the circumstances, if you want to save the plant, it's probably best to decant it from its current container and pot into something with drainage holes big enough to accommodate the rootball, using proper  potting soil. If  the rootball is soaking wet, don't water it; if it's not sodden, then give it a good soaking, allowing excess water to drain away freely from the base of the pot. Ongoing, keep it watered as necessary, in hopes that the plant will not get worse than it is now, and may recover.

Note also that clay soil per se is not necessarily bad for magnolia - so long as the ph is neutral to slightly acid, and the soil is fertile, having a high clay content isn't a major issue.