What is gain? In engineering the term refers to amplification and that is pretty much what the gain control on a radar does, it determines the amplification of the beam. But that isn't what it does from a pilot's perspective. What gain really does is change the focus of the beam, thereby changing the color calibration of what you see on the radar. Radar guru Archie Trammell says we should change the label from "GAIN" to "CAL" for calibration. The Honeywell Radar Training Course says we should think of it as a squelch control for the radar.
On most modern aircraft the gain control is preset to calibrated standards that make what you see on the scope easier to interpret and consistent with what the manufacturer intended. More about that: Procedures & Techniques / Radar - Calibration.
Yes, there are times adjusting the gain will give you extra knowledge of what lurks ahead, but as soon as you're done return the gain to its automatic setting.
What follows are quotes from the sources listed below, as well as my comments in blue.
Figure: Radiated beam width, from Honeywell Radar Training Course, Part 3, Slide 12.
Antenna Gain refers to how much of the energy leaving the antenna is focused into a particular direction. It is the ratio of the focused intensity to the average intensity.
Higher gain serves to narrow the effective beam width. With the gain properly set, ground clutter can be minimized and the tilt raised to miss ground obstructions, like cities:
Figure: Radiated beam width, from Honeywell Radar Training Course, Part 3, Slide 13.
You can think of gain as the squelch control on a radio. Turning the gain up focuses the beam to increase the energy in the center of the beam, turning the gain down decreases the energy. (This will be useful when looking for radar shadows.)
Figure: Radar signal amplitude, from Honeywell Radar Training Course, Part 4, Slide 18.
I recommend keeping the gain button pushed in normally so what you see on the scope agrees with the Radar Calibration convention. Starting with the calibrated screen, for example, you can slowly decrease gain to spot a shadow you wouldn't normally see. The following exposes a weather system you wouldn't have noticed before:
Figure: Shadow versus signal strength, from Honeywell Radar Training Course, Part 4, Slide 20.
You can also increase gain to help define the edges of a return.
Once you've adjusted the gain to reveal the hidden weather, you must return it to the calibrated position. If you don't, the radar will soon be lying to you, giving you colors that do not match what the radar is really seeing.
Portions of this page can be found in the book Flight Lessons 1: Basic Flight, Chapter 28.
Trammell, Archie, The Mysterious 'Gain' Knob, Business & Commercial Aviation, October 2010, pages 58-60.
Honeywell Airborne Weather Radar Training, Rev E, 12/09/02, Honeywell Inc. Commercial Flight Systems Group, Phoenix, AZ.
Honeywell Primus 880 Pilot's Guide, Pub. No. A28-1146-102-03, Revised January 2006, Honeywell International Inc. Commercial Electronic Systems, Glendale, AZ.