Maybe the problem isn't so much that Lisp is too easy, but rather that there's no central repository for Lisp modules? Everybody runs into the "I need a GUI environment" and doesn't have anywhere to search for the 30 other GUIs that have already been written and never published so they write their own. A centralized repository for these could help a lot, especially if it enforces documentation standards before accepting modules.
It's a cultural issue with all of the pre-Internet languages. Even titans like C and C++ lack a well defined repository outside of their stuffy standards committee defined standard libraries. CPAN showed how powerful a resource like that can be 25 years ago and almost every language since has shipped with something similar, but old languages never seem to have caught on.
Quicklisp exists now and largely addresses this issue. It can still be difficult to discover libraries sometimes, and not everything is in it, but it’s been a major step forward.
Some of them might be considered production ready, but that would depend on your expectations. They all pale in comparison with CAPI though, which is really good and I'd say one of the main reasons to use Common Lisp today.
I am sure there are several ones. Binding to GTK should be very easy through FFI. Myself, I have written a pretty large enterprise application based on Ltk.
It's a cultural issue with all of the pre-Internet languages. Even titans like C and C++ lack a well defined repository outside of their stuffy standards committee defined standard libraries. CPAN showed how powerful a resource like that can be 25 years ago and almost every language since has shipped with something similar, but old languages never seem to have caught on.