All the HP Ada 83 pragmas and attributes are recognized, although only a subset of them can sensibly be implemented. The description of pragmas in Implementation Defined Pragmas indicates whether or not they are applicable to GNAT.
In GNAT, the default floating-point format is IEEE, whereas in HP Ada 83, it is VMS format.
the package System in GNAT exactly corresponds to the definition in the Ada 95 reference manual, which means that it excludes many of the HP Ada 83 extensions. However, a separate package Aux_DEC is provided that contains the additional definitions, and a special pragma, Extend_System allows this package to be treated transparently as an extension of package System.