I think the answer he's looking for is a lot simpler
1L * 120 = 0.0024 L of bleach
50,000
That's for 1 litre of water
Question is the Gallons in your question is it Imperial Gallons or US Gallons?
If its imperial 0.0024 * 454.609 for your answer = 1.09 gallons of hypochlorite
If its US gallons 0.0024 * 378.541 = 0.908 gallons
Based off of calculations found in Chapter 8 of this textbook
https://dr6j45jk9xcmk.cloudfront.net...sinfection.pdf