The GN-13A is a regular-sized, side-view nixie tube, manufactured by ITT. It looks to be identical to the Hivac XN13, apart from the color of the anode mesh.
The digits on both this and the XN13 are noticeably larger than other tubes with the same size glass envelope – at 16.5mm.
This is an excerpt from an app note. The full text is available at Dieter’s site.