My girlfriends mom has huntington disease... If my gf and I have kids what is the chance they will have the disease too?

Genetic consult. It's great that you think ahead. Today there are genetic markers for huntington that can be studied & if your gf is positive she will both get huntington disease & potentially pass it to her kids (risk = 50% per conception). A consultation with a medical geneticist should be able to clear the air on this issue and analyse any risks from either side of the family.
Depends. huntington disease is a dominant trait. There it depends on whether your girlfriend actually has the disease. The disorder may take some time to become apparent. there are tests that can be done to determine is a person will develop Huntington disease befor they become symptomatic. If she does not have it, none of your children are at risk. If she does, the risk for each pregnancy is 25%