That's the point that's confusing. Especially in contexts where it's not obvious whether it's power-like or amplitude-like. It should be 40dB of power gain or 20dB of amplitude gain, with the context made explicit.

I don't see how it's confusing. Thanks to the scaling of voltage dB, you never have to worry, 10 dB of voltage gain is the same as 10 dB of power gain.