I don’t think there is an error in my table. Pure silica glass does have high modulus and, if free of surface defects is extremely strong (high yield strength). The reason for the ? under the “Strength” column is perhaps because no one has tested such a material with a guarantee that there were no surface defects. There are many different ways to make carbon fabric, and they vary in cost and strength. The epoxy Novolac in the table is probably typical for that composition. You may have in mind the modulus of a single carbon fiber, which is listed at the very bottom of the table and is very high. For both glass and carbon fibers, single fiber strength or modulus greatly exceeds that of their composite forms. Lastly, we are not talking here of carbon nanotubes, which I understand are many times stronger than bulk forms of carbon fiber.
Along with modulus and density, an important parameter in the construction of practical reed tongues is the yield stress, or the fatigue strength, whichever is less. Thus, many materials may have interesting E/Rho ratios, but they also may be unable to handle the stresses caused by vibration. Thus, only some of the very strong plastics might work okay.
In order to answer your question about making a gong, we would have to look at the governing (wave) equation for a gong. Acoustically speaking, a gong can be referred to as a “plate.”
In diversion, acousticians have labelled vibrating objects according to the nature of the restoring forces that maintain their vibrations. The simplest is a “flexible string,” or simply “string,” in which the only restoring force is caused by tension. There is no “stiffness,” or resistance to bending. Next up is a “stiff string,” in which the primary restoring force is one of tension, as well as a “small” contribution from stiffness. Here, the stiffness forces produce only a small perturbation of the vibration that’s produced in a flexible string. Next up are “bars,” which the free reed tongue is a member of. Bars are the opposite of strings in that the only restoring force considered is stiffness, or the tendency to resist bending, with no contribution from tension. The examples here so far involve one-dimensional objects. The concept of a flexible string can be extended to two dimensions in objects called “membranes,” which like the string, have only tension as restoring force. We also extend the concept of bars to two dimensions, calling them “plates,” and like bars, have only stiffness (bending) forces restoring motion. I believe a bell could be considered a three dimensional plate, although for the very large bells (going up to hundreds of tons), gravitational forces may influence their vibrations.
Getting back to your question, the wave equation for a plate involves the material factor, (1 – s^2)*(Rho/E), where s is Poisson’s ratio, defined by the ratio of transverse strain to axial strain when a material is compressed or stretched. An incompressible material undergoing no deformation (stressed within elastic limits) will have s = 0.5. In practice, s for steel is 0.27 – 0.3, for brass is 0.33, and for concrete is 0.1 – 0.2. Thus if you plug in these material values to the factor containing poisson’s ratio, you get for steel 0.00878, for brass 0.0169, and for concrete 0.0147, where the units are the same as I used in the original table, and for concrete, I used E = 5.8 Mpsi, and Rho = 0.087 lbsm/in^3. The result here suggests, without any other considerations, a gong made out of concrete, for equal geometries, should sound more like a gong of brass than like a gong of steel.
If I may ask, what kind of concrete gong are you thinking of? How big? Besides the simplified acoustic discussion here, there are other practical issues that should be considered in planning such an object. For large gongs, weight might present special concerns. I don't know about the ability of concrete to withstand vibrational stress. There are people who study these things, and you might search them out.