No need to gamma correct bump maps as they don't feed info into any color channels. I think the problem on the underside is stemming from the ambient lighting, if whatever is beneath it is light colored then I can see that happening when it throws its color onto the balloon.
SimplyMaya specialises in Maya tutorials. We offer over 1,000 individual Maya training videos, ranging from basic Maya tutorials through to intermediate Maya tutorials. Our tutorials are created by instructors with industry experience and are designed to get you up and running in Maya quickly without making it seem like hard work.