Saying 'X is a great piece of music' means exactly 'I like X a lot'. And if people say they think something is great yet they don't like it, or vice versa, they are pretty much talking out of the wrong hole. Also, I'm glad StLuke's is in this thread as he loves discussing this point.
As for Gentle Giant, I like a bit of prog (Pink Floyd, Focus, King Crimson, Rush, some Yes, Camel, Gong, a lot of the Canterbury bands) but some I just can't get into whatsoever. Gentle Giant, Genesis, Caravan, Van Der Graaf Generator, ELP and the prog-metal bands, with the possible exception of Tool, do absolutely nothing for me and I don't really know why. Genesis are especially puzzling as I don't really like any of their stuff but if I had to listen to some I'd choose the Duke and Abacab era over the Gabriel stuff. Saying that some of Steve Hackett's solo albums are decent.