RM,
The gas block on an AR15 is the cast block of aluminum that goes around the barrel at the gas port. The gas tube comes in the back side and the front sight base is an integral part of the top of it (on an A2 model). On some varmint and space gun types it's more or less just a block of aluminum that sits in the same place and serves the same function, but without the sight base on top.
Sorry, never heard of an "adjustable gas block", unless you are referring to one that is held in place with setscrews instead of being pinned in place. This allows you to adjust the cant of the front sight in relation to the rear....
Best regards,
Swampy