My grandma has als, what does that mean for me?

probably nothing: Less than 10% of ALS cases are familial. The vast majority are sporadic. Most familial cases are autosomal dominant, so you would probably expect other family members to have it. It can rarely be recessive.

Likely little risk: In 90% of all cases, there is no familial incidence. So, your risk is about 5 in 100,000, same as general population.