Go to The Engineering Toolbox web page and select "US gpm (gallons per minute)" from the Online Fluid Flow Calculator selection list. Enter a value and click Convert to have a pop-up window appear with conversions to all available units. If you prefer to convert the number yourself, the site also provides a quick conversion chart at the bottom. Simply locate "US gpm" from on the left side of the second table and read across until you find the header containing the unit you wish to use. Multiply the value in the intersecting box by the number of gallons per minute, and you will get your conversion.
Check out Volume Flow Rate Unit Conversion Calculator and enter the number of gallons per minute in the box labeled "US gallons per minute" or "Imperial gallons per minute," depending on which you're using. If you're in the United States, obviously use the former. If you're in the United Kingdom, you want to use the latter. Click "Calculate" and all the other boxes will fill in with the conversion values. All that is left is reading the value you want.
Try out the Online Flow Rate by Volume Converter from Convert-Me. The site works similarly to the previous one, except it provides more flow-rate choices. Simple enter your value in "U.S. gallon per minute (gal/min)" or "British gallon per minute" and click "Convert." All the other fields will instantly fill in with the appropriate conversion.