Help please. 29 weeks

Okay so I have a low lying placenta that is at the bare minimum of what they'll allow, I have placental bleeds, I'm anemic and my son weighed more than most babies at 27 weeks and 1 day ( 2lbs 6oz). Today I went for the glucose test, I failed and now have to take another but for 3 hours. What is gestational diabetes? Will they decide on a c section because of my low lying placenta and -if I am- gestational diabetes?

Only problems I've ever had was preeclampsia and hypertension with my first and my second was 8lbs 5oz with only a seizure.