The question is how to you make your white gravy? Do you use corn flour, egg white or what? The key to every kind of gravy is to dissolve the thing at first. For example if you are making it out of corn flour, first of all dissolve the required amount of corn flour in half cup of water. Then pour it in the rest of the dish while cooking at low heat. Also keep in mind that you would have to stir constantly.
The easiest way for me to make milk gravy is to dissolve Corn Starch in cold water, stirring well. I then pour it into the pan with the meat, and add milk. For me this works well. I never had much luck with flour. Corn starch dissolves more evenly in cold water.
If you are going to use flour though, use a strainer with a fine mesh, and pour the flour and water mix through the sieve, and it will remove the lumps that might be in the flour mix. Hope this helps.
If you are in the US, use Wondra flour. It's granulated specifically for making sauces. I swear by it!