gnu: Add freepats-gm.

* gnu/packages/audio.scm (freepats-gm): New variable.

gnu/packages/audio.scm

@@ -1980,6 +1980,51 @@ (define-public freepats
     ;; GPLv2+ with exception for compositions using these patches.
     (license license:gpl2+)))
 
+(define-public freepats-gm
+  (package
+    (name "freepats-gm")
+    (version "20210329")
+    (source (origin
+              (method url-fetch)
+              (uri (string-append "https://freepats.zenvoid.org/SoundSets/"
+                                  "FreePats-GeneralMIDI/FreePatsGM-SF2-"  version ".7z"))
+              (sha256
+               (base32
+                "19a1mp9yi33j2zff4mjvhrjz97dwwgjwzfdlf84j9xyydhx0crhc"))))
+    (build-system trivial-build-system)
+    (native-inputs
+     `(("p7zip" ,p7zip)))
+    (arguments
+     `(#:modules ((guix build utils))
+       #:builder
+       (let ((dir (string-append "FreePatsGM-SF2-" ,version))
+             (file (string-append "FreePatsGM-" ,version ".sf2"))
+             (out (string-append %output "/share/soundfonts"))
+             (doc (string-append %output "/share/doc/freepats-gm-" ,version)))
+         (use-modules (guix build utils))
+         (invoke (string-append (assoc-ref %build-inputs "p7zip") "/bin/7z")
+                 "e" (assoc-ref %build-inputs "source")
+                 (string-append dir "/" file)
+                 (string-append dir "/gpl.txt")
+                 (string-append dir "/cc0.txt")
+                 (string-append dir "/readme.txt"))
+         (mkdir-p out)
+         (copy-file file (string-append out "/FreePatsGM.sf2"))
+         (mkdir-p doc)
+         (for-each
+          (lambda (file)
+            (copy-file file (string-append doc "/" file)))
+          (find-files "." "\\.txt$"))
+         #t)))
+    (home-page "https://freepats.zenvoid.org/SoundSets/general-midi.html")
+    (synopsis "General MIDI sound set")
+    (description "FreePats is a project to create a free (as in free software)
+collection of digital instruments for music production.  This sound bank is a
+partial release of the General MIDI sound set.")
+    (license (list
+              license:gpl3+ ; with sampling exception
+              license:cc0))))
+
 (define-public guitarix
   (package
     (name "guitarix")