That sounds good, and is frequently recommended. Unfortunately, it doesn't work the way you might expect....a layer of gravel before putting putting the soil in.
In the ground, soil water rises above the level of standing water by capillary action. The upper limit of the soil water is known as the perched water table. The finer the particles of soil, the higher the perched water table with respect to the standing water. The action of the soil in lifting the water is similar to what happens within a wick or a sponge.
As an example, a sponge placed into a dish of shallow water will wick the water upward a certain amount, depending on the capillarity of the sponge, regardless of how much water is there. Capillary action depends on a continuous column or film of water. [In physics terms, it is a play between gravity and the cohesion and adhesion of water.]
When you water a pot of soil, the excess water drains out the bottom, through the required drain holes, and continues to drain until the capillarity of that specific soil mix (the sponge) prevents any more water from dropping out the bottom. The hight within the pot of the remaining water is its perched water table.
A layer of coarse material at the bottom of a pot "resets" the perched water table higher in the pot, as though the drain holes are higher, or closer to the top surface of the soil. So water is retained higher in the pot, compared to a pot of the same soil without the gravel at the bottom.
The outcome is that adding gravel to the bottom of a pot simply produces the effect of a shallower (and smaller) pot. It raises the perched water table.
Do a google search of "gravel in pots" or "perched water table" for many discussions of this.Washington State Univ. said:Since the stated goal for using coarse material in the bottoms of containers is to "keep soil from getting water logged,” it is ironic that adding this material will induce the very state it is intended to prevent.
https://puyallup.wsu.edu/wp-content/uploads/sites/403/2015/03/container-drainage.pdf
SUMMARY: Adding gravel to the bottom of a pot makes soggy soil soggier.
Bob