If I’ve done tayyammum to offer my Salat, do I have to perform Ghusl after Salat or not?
As long as the excuse for the tayyammum remains after the first tayyammum, and nothing occurred which will void Ghusl or Wudhu, then there is no need for a new tayyammum.
If the original excuse for tayammum no longer exsists, then for all the acts of worship where Taharah is needed, you must perform that taharah as per required (Wudhu/Ghusl).
This is the general view of all the Maraa’je.
Sheikh Mohammed Jafar Fathullah