I'll probably try to get into light sources later. I'm not very good at making these, but I was asked to make a current one. You might not get everything in this one so see
With gradient shading I sometimes use 2 different colored gradients of complementary colors. Example being if I'm using a blue gradient, I might use a orange gradient from behind or the opposite direction. I sometimes use gradients to create a light source from a glowing object too if the character is holding or near the glow. Sometimes the gradients have to be made manually especially if you're using SAI. Best way to make them is scribble a color using the marker and using the water tool to smear the color away.