Is it valid to suffice with standing under a shower for ghusl, and is it obligatory to wash only the areas that the water did not reach, or to repeat the entire ghusl if one remembers this after ghusl or after prayer?

The preference for pouring water to ensure it covers the entire body is indicated by what the Prophet, peace and blessings be upon him, said to Umm Salamah: "It is sufficient for you to pour three handfuls of water over your head, then pour water over your body, and you will become pure." If you are certain that a limb was not washed after the other limbs have dried, wash that limb only, then repeat the prayer. If you repeat the complete ghusl, there is no harm. However, if you are not certain, then you must disregard the doubts.