i have heard when light travels through a medium, certain things are not able to get through. certain subtle things are being filtered out, and in terms of sungazing, that is what you are getting, right? i would think it to be similar to filtering all of our smoothies through cheesecloth.
Glass will block (a portion of) UV-B radiation due the electron configuration in the glass (higher energy than UV-A which is closer to visible spectrum) and augment other wavelengths, undoubtedly. With something as esoteric as sungazing, you may be cutting invisible corners. The type of glass can make a difference as well.