My automatic watch used to gain 2 to 3 minutes a day, so I bought a cheap demagnetizer. Problem solved for $12. Hope it's the same for you. No I didn't buy that thing that measures the accuracy, I just observed for 24 hours.
This is what I bought but it's unavailable:
https://www.amazon.com/gp/product/B07CHKW4QT
It's from China so I'm sure there are tons out there that are similar.