The Amplitude and Compression > Fade and Gain Envelope effects function similarly but modify audio differently:
In the Editor panel, click the yellow envelope line to add keyframes, and drag them up or down to change amplitude. To quickly select, reposition, or delete multiple keyframes, see Adjust automation with keyframes.
Select the Spline Curves option to create smoother, curved transitions between keyframes, rather than linear transitions. See About spline curves for graphs.