Is it valid to perform ghusl if one forgets to wash the feet during ghusl, knowing that water is falling on them, and is it necessary to repeat ghusl in this case?

The entire body must be covered with water, along with the intention, when performing ghusl (definition) (ritual bath) after major ritual impurity (janabah (definition)). The water that falls onto the feet is sufficient. If, after performing ghusl, the person realizes that a part of their foot was not washed, they must wash only that part and are not required to repeat the entire ghusl, as continuity (muwalat) is not a condition for ghusl.