This is a bit of a general problem with PDL operations that return more than one result (for instance, I'm seeing this in the nearly-finished PDL::OpenCV, and I consider it somewhat of a problem), and I'm very open to ideas on a way forward.

pdl> ? gsl_sf_gamma
Module  PDL::GSLSF::GAMMA
  gsl_sf_gamma
      Signature: (double x(); double [o]y(); double [o]e())

    Gamma(x), x not a negative integer
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Note the two [o] parameters (i.e. outputs); the second one is the error, which here was a very small number. If you call the op in scalar context, you get the last output. In this case, that's arguably the least useful one, as shown by the OP's confusion. One idea is to just throw an exception for a multi-output op called in scalar context. Anyone got a view?