Extend Knob and Tube Wiring
First off, get yourself a new GFCI with screw terminals and do not use the back-stabs... those are trouble waiting to happen.
When you are adding the new receptacle down stream of the GFCI, it is not an extension of the old WIRING, it is an extension of the CIRCUIT. No restrictions there, so even if it is K&amp;T (I also question that), you are fine.
The potentially problematic issue if it IS K&amp;T is if it is also an MWBC (Multi Wire Branch Circuit), i.e. one where the neutral is shared by one or more other hot circuits. When you do that with a GFCI, it will cause nuisance tripping. But the fact that you HAVE a GFCI in there now, and we assume it has been working, would indicate it is not an MWBC.