Yep, the other Sippstrassi/Bloodstone series is based on the Arthur legend. The common link between the two is the stones and Shannow's not in them.
They're not a bad duo, but not as good as Shannow. Basically they give you more of a background into the stones themselves. If you enjoy Gemmell's style of writing then you'll enjoy them but they're not essential reading to the Shannow series.
If you want to go deeper into Gemmell's work, ty the Rigante series or the Waylander triology, both very good with Waylander similar to Shannow in some ways. However, only Shannow is set in a futuristic post apocalistic world, everthing else is more fantasy based but not heavy on the magic.
Hope you enjoy them.