php – Need help with Gravity Forms – Custom validation

I have a form created with gravity forms and I need a custom validation for one of the input fields. In my form there is a quantity count and you should be able to enter value from 10 to 5000, but the field should only accept numbers and those numbers should be multiples of 10. In other words, you should be able only to input numbers like 10, 20, 30, 40, 50 and so on to 5000. I’m really not sure how to make it so any tips will be greatly appreciated. Should I use JS for that or PHP? Thanks! :]