Who is supposed to give Ghusl to the body?

We should understand that Allah Ta’ala is our Creator and Master whilst we are His creation and servants. Therefore, He is fully entitled to create whomsoever He wishes as a male and whomsoever He wishes as a female. We should therefore be pleased with the decision of Allah Ta’ala in all matters, which includes the gender which He has chosen and selected for us.

Undergoing a gender affirmation surgery is not permissible in Islam due to various reasons, inter alia some are listed below:

1.. It is changing the creation of Allah Ta’ala which is from amongst the works of Shaytaan. Allah Ta’ala mentions in the Quraan-e-Majeed:

I will surely lead them astray, and certainly instil hopes within them. I shall definitely command them and they will slit the ears of animals. I will command them and they will certainly change Allah’s creation.

2. By undergoing gender affirmation surgery, one will be resembling the opposite gender. Such people have been cursed by Rasulullah Salallahu Alaihi Wasallam as quoted in the narrations mentioned below:

Ibn Abbas Radiallahu Anhuma states that Rasulullah Salallahu Alaihi Wasallam cursed those men who behave like women (by having their characteristics etc.), and those women who imitate men. It is recorded in another narration that Rasulullah Salallahu Alaihi Wasallam cursed those men who imitate women and those women who imitate men.

3. By undergoing gender affirmation surgery, one is expressing ingratitude to Allah Ta’ala, which indicates that one is not pleased with the decision of Allah Ta’ala.

Now, coming to your query, if one does undergo gender affirmation surgery, then he will still maintain the same gender as at the time of birth. If one was born a male, then he will remain a male and be given Ghusl by males, and if one was born a female, then she will remain a female and be given Ghusl by females.