Approved HSA expenses are the same as expenses that are approved for the medical expense deduction on schedule a, as listed in IRS publication 502. Medical expenses are deductible if they are used to cure, treat, or mitigate a disease or condition that affects the normal function of the body. Expenses for general health improvement or not deductible.
If you have a medical condition and the massage gun is a doctor recommended treatment, I would expect it to be allowable, and I don’t think you need a prescription. But it can’t just be for a general health benefit.
