Most stages of Genesis, Gentle Giant, and most stages of Jethro Tull
are all prog rock, trying to exlude them from being prog rock is not
reasonable in my view.
I was and am a huge fan of Gabriel-Genesis as well as a fan of the
next few albums after Lamb. I was am and a huge fan of Jethro Tull up
until Under Wraps (early 80's, I think). I had never heard Gentle
Giant until after being a big fan of Genesis and Jethro Tull and while
it took me awhile to follow their strange and funny ways Gentle Giant
(up until Giant for a Day) has joined those other two at the top of my
list. But I can't say to someone that if you liked one you would like
the other, they are each very different from each other and are
appealing in different ways.