How can you determine how bright a gas giant would be, especially if it has inhabited moons with an Earth-like atmosphere?
I found a good resource here https://www.youtube.com/watch?v=YRzJdg13geU&list=PLIIOUpOge0LuGoV8698AWjDcMYeB9OvEf&index=3 but basically that would depend on distance from the sun and orbit of the moon. Due to tidal heat and reflected light from planet such gaia moon would be further from the sun. With low orbit of something like 24 hours it could provide similar amount of light as sun. About 20000 more than moon but orbit that close would be inside radiation zone of giant. The further you go this will go down. Mind you that closer will cause planet side of the moon to be much warmer and further will make a day to last for several earth days both will have huge impact on ecology and climate (tidal winds).