Every minute, the Amazon River discharges approximately 3 billion (3,000,000,000) gallons of water into the Atlantic Ocean. Write the header and body of a Python secondary function named compute_amazon_gallons that accepts (as parameters) a given measurement in hours and a given measurement in minutes, and then returns (as the functional value) the amount of water in gallons that the Amazon discharges into the Atlantic during that designated span of time.
Recall that there are 60 minutes in an hour. Your Python code should not perform any user input or user output.