Hmm.. AFAICT the implementation of Master Reverb/Echo in GB doesn't seem to work properly [like other standard Send&Return effects]. I don't know if you're seeing the same behavior as I am, but here adding reverb with the Master Reverb slider in a track always seems to bring the sound of the dry track way up along with the added reverb, destroying the mix balance for the track. I don't know if it's a bug or a "feature", or just some weirdness on my system, but I'd avoid it -- instead of dialing up reverb with the Master Reverb slider for each track, my suggestion is to leave those all the way down [and leave the Master Reverb slider in the Master Track all the way down as well], and use whatever reverb/ambience knob [usually over on the right] is provided in the Smart Controls [graphic] panel itself. Those seem to be for individual channel reverbs and they seem to work normally.
Generally, mixing functionality is the weakest aspect of GB, and probably one of the main reasons many people eventually upgrade to Logic..