I've looked into it a bit. The answer appears to be… sort of. It depends on your definition of "modern," how many hoops you're willing to jump through, and which dialects of Lisp you're willing to use. These are some of the more promising options I found.

For Common Lisp, one of the longer-lasting and best supported options appears to be CAPI, which comes with LispWorks (http://www.lispworks.com/products/capi.html), but that costs a grand and is not open source, so that's kind of out of the picture for most people. If you're looking for commercial option that will just work, though, this might be worth investigating. (They seem to have recently released a "hobbyist" version that is only $500, but you can only use it for "non-commercial and non-academic purposes," so I'm not really sure how useful that is.)

Ceramic (http://ceramic.github.io/) lets you use Common Lisp to make desktop web apps using Atom's framework (http://electron.atom.io/). While that comes with all the disadvantages of making desktops apps with HTML+CSS+JS, it seems to be one of the more professional and easier to set up options.

If you're using a Java implementation of Common Lisp, such as ABCL (http://abcl.org/), you can presumably use any Java GUI framework. I've heard, however, that using Java libraries is significantly more awkward in ABCL than, say, Clojure, and that you might need to write some Java code to bridge the gap. The main attempt to get that to work I've seen appears to be this framework ( https://common-lisp.net/project/snow/ )—I'm unaware of any applications actually written like this.

Clozure CL (not to be confused with Clojure) has bindings to Apple's Cocoa framework (http://trac.clozure.com/ccl/wiki/CocoaBridge). Apparently there's a fairly battle tested cross-platform implementation of Cocoa that you can use to make it run on Windows and Linux too, but I'm having trouble finding a link to that right now. I think it was the framework used to port Comic Life to Windows, if that helps. Regardless, though, this sounds like a pretty scary option for someone without prior Cocoa experience.

Most of the more practical looking options I found seemed to be for Scheme.

Racket (https://racket-lang.org/) comes with a GUI toolkit that was originally forked from wxWidgets. The IDE is built in it, among other things. It has some quirks that wxWidgets doesn't, though—for example, all the widgets are native except for all text entry controls, which makes accessibility… interesting. (Also, on my computer, at least, they don't quite cooperate with high DPI displays.)

Chicken Scheme apparently has good bindings to IUP (http://wiki.call-cc.org/eggref/4/iup, http://webserver2.tecgraf.puc-rio.br/iup/), a fairly lightweight framework that uses native widgets on Windows and Motif/GTK on Linux/Unix/OS X. I wouldn't really call it modern, but it does look fairly convenient—the Scheme API seems to basically let you define GUIs in structured s-expressions (http://wiki.call-cc.org/iup-tutor#porting-some-examples-from...). (Also, I'm pretty sure it does support Unicode, and that the IUP site is just out of date.) This is the option I'm leaning towards learning.

Kawa Scheme runs on Java, providing the same benefits as ABCL, but it seems a little more battle tested, and provides native support for making Android applications (http://www.gnu.org/software/kawa/Android-view-construction.h...). It's used to, among other things, power MIT App Inventor. (You can actually unzip App Inventor projects and find Scheme files inside, which is kind of fun. I kind of wish the App Inventor team would make the framework more accessible from Scheme. :p)

And, of course, if you're willing to settle for dialects of Lisp more distant from Common Lisp or Scheme, you can use Hy and any Python GUI framework (http://docs.hylang.org/en/latest/), or Clojure and anything Java.

Hopefully there are some good ones I missed, because Lisp-like languages seem like they would be pretty cool for GUIs.