Ahh okay, I'm sorry to say it is more complicated than it seems since this is an SRPP stage, the formula from the Tube CAD article is for a cathode follower (actually the cathode follower output impedance can be fairly accurately estimated by 1/gm). You will find different formulas around the web for the output impedance of SRPP, for identical triodes and the lower cathode resistor bypassed (like in GOTL), I came across this formula by a knowledgeable chap on diyAudio.
Zout = (Ra^2)/(2Ra+(mu+1)Rk)
Where Ra is the plate resistance and Rk is the upper triode cathode resistor, 250ohm in GOTL. For a 245ohm plate resistance, that would give an output impedance of around 20ohm but hard to say how accurate that is as the plate resistance is also dependent on the bias point and the GOTL will not bias a 6528 as they are referencing in the datasheet.
I measured the plate resistance of a 6528 on my tracer for around a 100V / 100mA bias point (ballpark of where it would be in the GOTL), got a plate resistance of 330ohm, which by the formula above would make for an output impedance of 34ohm, so that might be a little closer to the mark.
I actually have a LTSpice simulation of the GOTL circuit, calculations in LTSpice tend to be pretty accurate. Unfortunately I don't have a 6528 model so I can't say what the output impedance would be using it. I do have a 6336 model though, output impedance is around 65ohm in my simulation.
The absolute best way to find the output impedance though is to measure it! It is very easy to do, you just need an old headphone cable you don't mind cutting to access the wires, two different resistor values, two alligator clips, and a digital multimeter. If you or anyone else is interested in how that is done, I can describe the process.