I adjusted your definition code, but I'm not totally sure what you want to do with it. But here is the modified code.
GPA = input("What is your GPA: ")
def GPAcalc( A, B, C, D, F):
if (GPA == "A"):
print("4")
elif (GPA == "B"):
print("3")
elif (GPA == "C"):
print("2")
elif (GPA == "D"):
print("1")
elif (GPA == "F"):
print("0")
elif (GPA != "A", "B", "C", "D", "F"):
print ("Invalid")
print("Your GPA score is: " + GPA)
Would you mind explaining what you want to do? It would help.
:)