Volume, flow rate, and time are one more three-way relationship - know any two and the third follows. A plain converter can't help here at all, since "gallons per minute" isn't a single unit, it's a rate connecting volume to time.
A flow rate isn't really one unit - it's a ratio of volume to time, which is exactly the kind of relationship a plain unit converter structurally can't handle. Once you know any two of volume, flow rate, and time, the third is fixed: a 5,000-gallon pool at a typical 10 GPM hose takes about 500 minutes, or 8.3 hours - long enough that most people underestimate it badly by eye.
Garden hose flow rate varies a lot by diameter and water pressure - a narrow 1/2" hose commonly runs 5-9 GPM, while a wide 3/4" hose can push 15-25 GPM. If you don't know which you have, the bucket test above is the standard, genuinely reliable way to measure it directly: time how long a known container takes to fill, and the flow rate follows.