Hello. From the information provided and the attached pictures, you are most likely dealing with Fungus gnats. These gnats infest in generally the soil, but also the potting soil in pots, containers, etc., feeding mainly on decaying organic matter. As the development of fungus gnats is also determined by the high, persistent humidity in the potting soil (also favored by the presence of pebbles), it is recommended to pay attention to the frequency of watering (letting the potting soil dry slightly between two waterings while respecting the watering requirements of the plant), as well as the removal of the pebbles from the pot. If the degree of infestation is high, you can choose to replace the first few centimeters of the potting soil (the flies usually develop there) with a new, fresh one ().
In addition, to capture them (thus reducing the population of fungus gnats), you can consider using an adhesive trap such as or . Fungus gnats are attracted by the color of the paper and stick to the special adhesive.