How to test a thyristor and distinguish an ordinary one and GTO ?

Hello~ all

Nice to be here! I am a student who now learning the electronic course at college. And is there anyone can help me?

Question one: 

I am checking a large high voltage device.  It has a bunch of BIG thyristor packages (Semikron, SKKT 106/18 E, Dual Thyristor Module, 600A 1800V). 

I'm testing them on a Fluke 87V multi-meter, and they all read EXACTLY the same.

In Diode mode, they all read 0.018v between gate and cathode in both directions, open circuit on all other pins.

In Resistance mode, they all read 18.1 ohms between gate and cathode in both directions, open circuit on all other pins.

I would assume that this means that the devices have all failed in exactly the same way with a gate to cathode short, but it would be very odd if this was the case.

Does anyone think that these devices have failed with a gate to cathode short, or would you expect this behavior on a multi-meter?

And how about the 3TA80GK03NB/04? This is also a thyristor module, but with three phrase.

Question two:

A thyristor, I know, is a four-layer PNPN structure, with an anode on the first P section, a gate on the second P section, and a cathode on the second N section. This simple structure suggests that any thyristor ought to be possible to turn off, by routing all of the anode current out through the gate, making the cathode current go to zero, thereby unlatching the thyristor. What makes a GTO thyristor special? Is it just an ordinary thyristor but with specified characteristics for this mode of operation? Or is there some different silicon structure inside of it that makes it work fundamentally differently? How do I know what kind of thyristor should buy?

Regards!