Can I simply increase gain of GC?

Sure, you can very easily increase the gain by changing the feedback resistors. Just keep in mind that by increasing the gain you are boosting the noise floor by the same amount, and this is a constant background hiss that you may hear through sensitive drivers like compression driver tweeters. The noise comes from the DAC noise floor, not the amp, so there is little you can do about it.

The best solution is a variable gain like an active volume control board after the DSP and before the amp. Then you can "turn it up" when you want to and also have the ability to turn down the gain, and also the noise floor, for quiet listening.
 
Here's a different gain option for your GC, with this schematic from Philfr
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*You can also add a 180p cap from -in to +in. And you can add the full T/S output parts (only half are shown on the schematic).
 
*You can also add a 180p cap from -in to +in. And you can add the full T/S output parts (only half are shown on the schematic

Actually, I did try adding 220p across the inputs, and it drives amp into some HF oscillation, even with output RC network present. I could not catch it on the scope nor hear anything, but I could feel the heatsink heating up much more.