There is a SYS$TEMP on VMS, SYS$ logical names are supposed to be
reserved to HP, in fact anything with a $ in it. One could argue that
GNAT is the HP standard Ada compiler on VMS and is entitled to use it. I
can ask for HP's opinion and report back.
That's not the way I'd interpret the "reserved to DEC [HP]" for logical
names with $. I'd interpret that as meaning that you aren't supposed to
DEFINE such names, not that you aren't supposed to USE them for their
specified meanings. I don't see why putting temp files in SYS$TEMP
would be a problem for ANY application.