Praise be to Allah.
What is required when doing ghusl is to make the water reach every part of the body. If it is possible to make the water reach the scalp and go through the hair and so on, then he does not have to rub it, but if it is not possible to do that without rubbing it, then he has to rub it.
What is required of the one who is doing ghusl is to pour water over all of his hair and make it reach the roots of his hair, whether his hair is thick or thin. If he does that then his ghusl is complete.
It was narrated from ‘Aa’ishah (may Allah be pleased with her) that she said: When the Messenger of Allah (blessings and peace of Allah be upon him) did ghusl for janaabah, he washed his hands and did wudoo’ as for prayer, then he washed his body, then he ran his hands through his hair, until he was sure the water had reached the scalp. Then he poured water over his head three times. Then he washed the rest of his body.
Narrated by al-Bukhaari, 264
It says in al-Mawsoo‘ah al-Fiqhiyyah (13/19): The fuqaha’ are unanimously agreed that making the water reach all of the hair and skin is one of the obligatory parts of ghusl. End quote.
It also says in al-Mawsoo‘ah al-Fiqhiyyah (26/105): The fuqaha’ are unanimously agreed that it is obligatory to make the water reach all the hair of the head, down to the roots, for both males and females. End quote.
Shaykh ash-Shanqeeti (may Allah preserve him) said: It is more appropriate to say that rubbing is not obligatory, but if the water is little, and making the water reach all of the body can only be done by rubbing, then it is obligatory in the sense that whatever is essential to doing an obligatory act is also obligatory. End quote from Sharh Zaad al-Mustaqni‘.
And Allah knows best.