I read it in high school too and found it very thought-provoking; I still think about some of its ideas. You'll definitely get more out of the book if you put effort into it, but it's not heavy like reading Knuth or SICP or anything on category theory, which I never made it all the way through.
I'm not sure how well GEB has aged though since it seemed very tied to a particular point in AI history. Has anyone read it recently?
I read it a few years ago, I think it's aged very well. The parts about art, music, and math are pretty much timeless, and I think the overall theme of AI is general and high-level enough to not really be tied to one era of computer science. You're not going to learn much that can be practically applied to contemporary AI techniques, but that's not really the purpose of the book.
Also read it in high school. Could only read about 2-3 pages at a time before brain was overwhelmingly & delightfully engaged and had to go have a long think.