As an optional step, blend your gochujaru in a spice grinder. Gochujang pastes I have tried are mostly smooth (I daresay the chilli breaks down during the fermentation). This is totally optional but an idea if you're keen to replicate the consistency as well as the taste.
Add the miso paste to a small saucepan and pour over the hot water. Whisk to combine then set aside for the miso to dissolve.
Once the miso has dissolved, add the gochujaru to the mix along with the maple syrup. Whisk to combine, then place over a low-medium heat. Cook for a few minutes until the paste has become thick.
Taste the paste and adjust according to your preferences. If you find it's not salty enough, add the gluten free soy sauce here. You can also thin it out with more water (which you might need to do if you add more chilli flakes, see notes).
If you're adding it, add the kimchi brine to taste. Allow the paste to cool before decanting it into a medium sterile glass jar. Store the paste in the fridge and use it within a week or two. Discard it if it develops mould or tastes different.