Normal compressors do effectively do gain adjustment - it's not really the same as a typical amp since their core function only reduces gain, then makeup is applied to the entirety to compensate - but yes the result is effectively gain adjustment.
As for doing it "without regard for the frequencies present", if you compress a mix with a base guitar & high vocals, the impact of the compression on the base will be different than on the higher notes. This is aside from (/in addition to?) attack & release applied on a per-track basis & more just about the natural effect of dynamics within frequency ranges.
Are you referring to psychoacoustic effects? Something like perceived loudness is only determined by waveform amplitude, but also affected by frequencies present? Or maybe the vocal and bass parts have transients at different times causing the other to be attenuated when it was already relatively quiet?
Other than that I'm not sure what else you could mean. Maybe we've been using different compressors. I've used a small handful of hardware and software compressors and haven't found them to sensitive to spectral content when used "normally". Meaning no extra filters or side chain configuration.
Normal compressors do effectively do gain adjustment - it's not really the same as a typical amp since their core function only reduces gain, then makeup is applied to the entirety to compensate - but yes the result is effectively gain adjustment.
As for doing it "without regard for the frequencies present", if you compress a mix with a base guitar & high vocals, the impact of the compression on the base will be different than on the higher notes. This is aside from (/in addition to?) attack & release applied on a per-track basis & more just about the natural effect of dynamics within frequency ranges.