GPU Clock = Base clock... stock clocks with no 3D load.
Boost Clock = MINIMUM BOOST clock (assuming temp/TDP isn't breached). There are many boost bins well above the boost value listed.
As far as the lower and upper... one is stock and remains static (the lower) and one is dynamic (changes as you set the clocks in say MSI AB). But you can boost over that value (and it usually happens). Assuming that is what you meant by lower and higher....