Approach:

Read the file.
Store the content of the file in a variable.
Use the count() method with the argument as a letter whose frequency is required.
Display the count of the letter.

# Program to get letter count in a text file

# explit function to return the letter count
def letterFrequency(fileName, letter):
# open file in read mode
file = open(fileName, 'r')

# store content of the file in a variable
text = file.read()

# using count()
return text.count(letter)


# call the function and display the letetr count
print(letterFrequency('gfg.txt', 'g'))