Ltk is quite popular, very portable, and reasonably well documented through the Tk docs. Installation on SBCL is as easy as saying:
(require :asdf-install)
(asdf-install:install :ltk)
There’s also Cells-Gtk, which is reported to be quite usable but may have a slightly steeper learning curve because of its reliance on Cells.
EDIT: Note that ASDF-INSTALL is integrated this well with SBCL only. Installing libraries from within other Lisp implementations may prove harder. (Personally, I always install my libraries from within SBCL and then use them from all implementations.) Sorry about any confusion this may have caused.