Well the so called "baby babylon" barrel (151 feet long) was about 100 tons. We can scale the weight down a few notches for the smaller 100 feet long test barrel we've seen (more than a third?), and down a little more for a weapon built with modern technology as opposed to 1980s Iraqi engineering, along with a further slight decrease by improving the physics of the shell that can improve range with a smaller gun (such as the suggestion of ramjet powered shell?).
Baby Babylon had a rather thin barrel, and was supported by the ground.
A Chinese supergun would need to have a thicker barrel in order to elevate the barrel/be durable enough to fire hundreds of shells without being refurbished. And the largest Chinese mobile ICBMs, like the DF-31A or even the DF-41, probably weighs in at no more than 50 tons.
A supergun, with TEL, weighing 100-150 tons at minimum would require strengthening the tunnel floors (granted, we don't know what the load bearing is for the average tunnel, but I can't see the 2nd Artillery gold plating them that much).
We'll then have to add some weight because a gun obviously is never just the barrel. However such a contraption would probably not involve an automatic or even semi automatic reloading mechanism that is organic to the gun, and there may indeed be an entirely different vehicle to provide semi automatic reloading, so there's some more weight we can cut off.
I'm not sure how heavy such a complete gun would be, but I don't think it'll be several hundred tons heavy.
You're right, it could be just a couple of hundred tons. The problem is more that putting elevation and traverse machinery onto the TEL is that you'll most likely widen and heighten the vehicle, meaning that it might not fit into the smaller tunnels.
Now, the question is whether said weight can be established on an existing mobile platform.
The largest WS series special purpose vehicle (TEL used for various PLA IRBMs and ICBMs) which is the WS51200 can carry a load weight of 120 tons
I believe that may be enough for a road mobile 100 ft gun. If there is also a road mobile reload vehicle supporting it the weight for the gun itself may be further lowered.
It could, depending on how much of the elevation and traverse machinery you offload from the TEL. But I doubt if most of the tunnels can handle the WS-51200 (modern Chinese ICBMs don't need that big a platform).
Also, why do we even need a TEL? We could say the gun and reload vehicle are only towed, thus opening up a much wider array of vehicles that can potentially simply tow such heavy loads instead.
There's obviously enough space for a TEL to carry a gun that long, if you let the forward barrel poke out a little.
If you don't self propel the supergun, it will take a lot longer to set up, reload and disassemble. A longer assembly/disassembly time gives a longer window for the LACMs and other unpleasantness to arrive at the firing position. A smaller TEL will also decrease mobility and lengthen the preparation time.
