I just discovered an oddity in GAP and am trying to figure out why it is
the way it is: Subgroup(g,elts) works only when g is a parent group.

As far as I can see, this means that when I write programs, where I
don't know whether g is going to be a parent group or not, I should
actually use Subgroup(Parent(g),elts) instead of the above. But why
does Subgroup not automatically use the parent of g, if that's what it
needs, rather than making me explicitly say to use it? Or am I going
to get into trouble using Subgroup(Parent(g),elts)?

David Sibley | "Accurate reckoning. The entrance into knowledge
Amateur radio NT3O | of all existing things and all obscure secrets."
sibley@math.psu.edu | -- The Rhind Papyrus