What is the use of the gain control tool?

Someone thought it'd be handy. It's like a multiband compander. The subbands are "normalized" independently and then further transformed. This is done to minimize pre-echo I believe. (The quiet high frequency parts before an attack are boosted while encoding and attenuated in the decoder so that the quantuzation noise also gets attenuated)

In a sense it's similar to TNS. The difference is: TNS is applied on the completely transformed data and this GAIN thingy is done between two filterbank stages (hybrid filterbank).