What is the GUI that can be used for Mac and Windows in common lisp?

"First of all, I don't think there is a common processing system common and multi-platform GUI toolkit at the moment (May 2015) that ""this is the GUI."""

"So, at present, I think ""there are a lot of problems = The overhead of research and learning is getting bigger, so I'm in trouble."""

It depends on the tool you create, but if MacOSX and Windows share the same toolkit and the learning cost of the toolkit itself is low, it would be around Ltk.

• Practice GUI at Ltk - Othello

Other things that can be used in common with MacOSX and Windows, and where deployment costs are low, are the CAPI of LispWorks and Common Graphics of Allegro CL.
Both LispWorks and Allegro CL are commercial processing systems, but depending on the application you create, there is room for consideration.
Both are rather generous with Windows.
It's a commercial processing system, but shipping a single application is a relatively realistic choice along with the ability to create executable files.

If the overhead of GUI toolkit learning is not taken into account, there may be CommonQT, cl-gtk2-gtk.
If something goes wrong with bindings in these toolkits, you will often have to know the toolkits themselves and know how to bind them.

Common Lisp has always had a native Lisp GUI environment called CLIM, which is very Lisp-like, but it is not easy because it is not well maintained at present.

If it's completed on a certain screen (such as a game), there may be a relatively large number of people using lispbuilder-sdl.

I think it depends on the tool you create, but Ltk is easy and good enough to display the screen and press the button or type in characters.