Use gfm-mode instead of plain markdown-mode

core/prelude-packages.el

@@ -170,8 +170,8 @@ PACKAGE is installed only if not already present.  The file is opened in MODE."
 ;; markdown-mode doesn't have autoloads for the auto-mode-alist
 ;; so we add them manually if it's already installed
 (when (package-installed-p 'markdown-mode)
-  (add-to-list 'auto-mode-alist '("\\.markdown\\'" . markdown-mode))
-  (add-to-list 'auto-mode-alist '("\\.md\\'" . markdown-mode)))
+  (add-to-list 'auto-mode-alist '("\\.markdown\\'" . gfm-mode))
+  (add-to-list 'auto-mode-alist '("\\.md\\'" . gfm-mode)))
 
 (when (package-installed-p 'pkgbuild-mode)
   (add-to-list 'auto-mode-alist '("PKGBUILD\\'" . pkgbuild-mode)))