I've thought it out a bit and realized that the gas building up in the etchant bottles was hydrogen
Not necesarilly, hydrogen peroxide decomposes to water and oxygen.
HCl alone is not enough to dissolve copper, as copper is too noble to be dissolved just in acid (no matter how strong). You need some additional oxidizing agent. It can be an acid itself (that's the case of nitric acid), it can be something else, like Fe3+
or persulfate, or hydrogen peroxide.
Search forums, I think chemistry behind copper etching was already discussed before.