Using applyGain to add additional gain, rather than set the gain?

Audio Plugins
2

That is the difference between gain and level…
Gain is a factor, that multiplies the values.
If you express a gain in level, it is a logarithmic value and is adding to the existing levels.

Applying a gain of 2 (factor) will add 6 dB to the level. Applying factor 4 is adding 12 dB and applying factor 8 is adding 18 dB.

HTH