Gravex 20 EW Fungicide does not require watering in on the product label, you should use sufficient water to cover the area as needed for your target disease.
The rates for Gravex 20 EW Fungicide will vary depending on the disease per the product label, but for the most part they are 1.2-2.4 oz per 1k square feet. Unfortunately the product does not give a per gallon rate as they expect you to calibrate your sprayer in order to determine how much water to use. Here is a video to assist with that ;