What is the ruling on putting gel on the hair, does it prevent water from reaching it, and is it applied after ablution and wiped over in the next ablution?

Gel of animal origin is prohibited if its source is an animal that is not permissible to eat or is a carrion (dead without proper slaughter). It is permissible if its source is an animal that has been ritually slaughtered (dhabihah) and is permissible to eat, provided it causes no harm.

As for the ruling on gel during ablution (wudu'): If it prevents water from reaching the skin, some scholars have permitted it, such as Sheikh Ibn 'Uthaymeen, because wiping the head is based on the principle of ease (takhfif), and what is required is wiping, not washing. This is unlike major ritual impurity (janabah) and menstruation (hayd) washes, where it must be removed.