There is no difference between an RF gain control and AGC. AGC is just Auotmatic Gain control instead of manual gain control.
The only way to copy a weak signal adjacent to a strong one is to prevent the strong signal from affecting the AGC, and the only way to do that is by using a narrrow filter or notch filter (at IF, not audio) to attenuate the strong signal. You can use a wide (SSB) filter to see all the stations in the passband, and then use Passband Tuning or IF Shift, or a narrow filter (at IF, not audio!) to narrow in on the station you want to work if it is one of the weaker ones. You do not need to do anything for the strong signal unless it is overloading your front end and then you can switch in attenuation and switch it out again when you are finished. Many people have experienced a weak PSK31 signal disappearing or waterfall darkening when a strong signal comes on. This is because the strong signal is reducing the gain (and therefore the noise background), just the same as if you manually reduced the gain, and generally the only cure for this is using narrow filtering. Some receivers, designed specifically for PSK31, such as our latest PSK-20, do not use AGC, but distribute gain in such as way that it can copy weak signals adjacent to strong ones, without distorting the last IF stage or detector, but few transceivers can do this. A "dual-loop" AGC system may help and some high-end transceivers have this. 73, Skip KH6TY
