There are two versions:
1. The free version operates as a simple Gain controller, with just one knob comprehended between the fixed values of -96dB and +24dB.
This version will essentially act as your typical volume fader. The knob can be automated.
2. The paid version contains the Gain controller and also two other knobs: a Minimum and Maximum value controllers, which allow to define the values between which the Gain knob operates. These controls can be automated.
These extra knobs become useful when you need to do things like drawing automation or fader-riding.
That's because, for large minimum and maximum values, small changes in the gain knob or automation line results in broad and imprecise gain changes.
But if you need to do some rigorous gain changes, you can set enter small minimum and maximum values.
In the image below, the plugin was set to operate between -6dB and +6dB. As you can see, the large dip in the automation curve corresponds only to an attenuation of -5.2dB.
SLGain is a VST2 and VST3 gain/volume plugin for Windows.