Volume level is what you are hearing.
Gain level is how much amplification is being given to the signal.
If you have a very low-level input signal, even a large amount of gain may only achieve low volume.
On the other hand, if you have a very hot input signal, you might only have to just barely move the volume knob to get quite loud volume.
Regarding cartridges, the gain setting on the phono preamp should correspond to the output level of the cartridge, so that the signal to the linestage is at a useful level for the volume control to operate in, and be at a similar level to the rest of your source equipment. Low output cartridges will need higher gain, and high output cartridges will need lower gain settings at the phono stage.
Hope that was what you were looking for.
Gain level is how much amplification is being given to the signal.
If you have a very low-level input signal, even a large amount of gain may only achieve low volume.
On the other hand, if you have a very hot input signal, you might only have to just barely move the volume knob to get quite loud volume.
Regarding cartridges, the gain setting on the phono preamp should correspond to the output level of the cartridge, so that the signal to the linestage is at a useful level for the volume control to operate in, and be at a similar level to the rest of your source equipment. Low output cartridges will need higher gain, and high output cartridges will need lower gain settings at the phono stage.
Hope that was what you were looking for.