Thank you for the opportunity to help you with your question!
1 cubic foot equals 7.48 gallons. Since you have 8 cubic feet and want to convert them to gallons, you must multiply 8 by 7.48 and that's how you receive the number of gallons you have, which comes out to 59.84 gallons per second.
Next, in order to convert the number of gallons per second (gps) into gallons per minute (gpm) you just need to multiply the number of gallons per second, 59.84, by 60, since there are 60 seconds in a minute. This gives you 3590.4 gallons per minute.
So overall, all you need to do is multiply 8 by 7.48 and then multiply that product by 60 to get your final answer of 3590.4 gallons per minute (gpm)
Please let me know if you need any clarification. I'm always happy to answer your questions.