I just bought a succulent garden from home depot the other day and I've left them on my shelf right in front of my window for sunlight. But I noticed a lot of leaves falling off of the tallest succulent in the garden. A few also fell from the spikey one also. I want to know how I can keep them from dying but I'm not sure if they're suffering from too much water or not enough, because I have no idea what people did to them before I got them.

Two thoughts:

Is the gravel on the surface of the pot glued together? If so, break it up and pick it off. That will hold moisture in. My experience with Home Depot is that they don't care for plants at all so I suspect over-watering may not be it.

Are the plant affected plants the ones closest to the window? If so, it could be sunburn as these plants are grown under lights. Sunshine is a foreign subject so plants need to be introduced slowly to sunny conditions.

Agree with removing the gravel. I don't see any sign of sunburn in your pics. & would keep them in my sunniest spot if inside.

If you find peat under the rocks, it's going to be hard to keep these plants healthy. Peat + succulents = rotting roots (overwatering, even if you don't literally add much water. The roots can't breathe in that stuff.) If the pot does not have a drain hole in the bottom, the difficulty increases.