I agree with Spencer on the settings he described.
Regarding the technical aspects of loading, the cartridge generator is designed to operate into a "load" which it was intended to be linear with. This is a balance of the voltage and current output levels of the cartridge. Lower loading increases current output, and higher loading decreases current output. These factors actually affect the linearity of the cartridge generator, and will affect frequency response and control. Generally in an MC cartridge, making the load higher than specified will cause a rise in high-end frequency response, making is "zippy" sounding(due to reduced hi-freq damping). Making the load too low can cause the cartridge to become out of control and sloppy, especially in the bass. Sticking close to the manufacturer's recommendations for loading is appropriate, but small deviations may be better sounding on your system, so experimentation in a small range can be good.
Gain, is the amplification of the small signal output of your cartridge generator. It is typically described in the logarithmic db scale. The phono stage requires a certain level of input signal strength, in order to finish its amplification(gain) stages, and send it to the linestage, so that the desired SPL can be reached without dialing-in too much gain in the linestage, which can increase noise floor, and limit dynamic headroom. If the input signal is too weak for the gain setting selected, then you have to turn up the volume to an abnormally high level, just to achieve listening levels. In many cases, a weak input signal combined with an insufficient gain setting can make the music sound lifeless. Overly high inputs to the phono stage can overload the phono input stage, and cause distortion, and too much output from the phono stage into the input of the linestage can also cause overload distortion. You should match the output of the cartridge to the available gain settings on the phono stage, to get the desired result. For the Shelter 501, a phono gain of about 62db or higher would be appropriate, and I use about 66db on mine. Load for the 501 is 100 ohms, but it can vary by a few percent for optimization, if you want to experiment with it.