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 ;
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.