Almost nothing you laser is nontoxic. It’s all about good ventilation. The basic rule of thumb is: don’t breathe in what the laser process creates, and don’t eat anything that is in the machine or that comes in contact with the lasered materials.
As for exactly what glowforge’s secret sauce is… well, that’s secret and they are’t telling. Our best guesses have been polyurethanes or shellacs, both of which are safe for the machine to laser, but neither of which I’d recommend breathing the fumes that are generated when lasering. I’m pretty sure glowforge will tell you not to breathe in the smoke and fumes that are generated from proofgrade. (heck it’s probably in a user manual somewhere, anyone got a link?)
Again, though, I wouldn’t recommend breathing in any of the fumes that you make, regardless of material. Combustion byproducts are rarely good for you.