If both parents are hla-b27+ would that guarantee all offspring to have the gene? if so would only 1 positive parent make it a 50% chance?

No: Mendel's laws. Each parent has one copy of this and one copy of another. Let us assume neither parent has two copies. There is a 75% chance. Now, why do you ask? HLA-B27 is common, and the only thing that makes it troublesome is that it gives a few percent of folks joint problems. It is not a reason to forego parenthood. Ask whether your sacroileitis might benefit from today's new Rx's.