Question: Must I move my detached gums in the ritual bath (ghusl) in order for the water to reach underneath?
Answer:
In the Name of Allah, the Most Merciful and Compassionate
You are not obliged to move your detached gums in the ritual bath (ghusl) in order for the water to reach underneath [Tahtawi/Shurunbulali, Hashiyat Maraqi al-Falah].
Water Reaches
Even though you may assume that water does not reach underneath your gums – the basis is that it does. Water is naturally thin and permeating. Since this is the basis of understanding one would need reasonable surety to assume otherwise [Ibid.].
Due to this, you should ignore any assumptions that the water does not reach. Merely swish the water around your mouth and consider that integral complete.
Hope this helps.
Allah knows best
[Shaykh] Yusuf Weltch
Checked and Approved by Shaykh Faraz Rabbani